EARTH 

SCIENCES 

LIBRARY 


U'-C 


THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

GIFT  OF 

William  E.  Colby 


CAMBRIDGE   GEOLOGICAL   SERIES. 


PETKOLOGY  FOE  STUDENTS 


SonDon:    0.  J.  CLAY  AND  SONS, 

CAMBKIDGE   UNIVEESITY  PEESS  WAREHOUSE, 

AVE  MAEIA  LANE, 

.     AND 

H.   K.   LEWIS, 
136,    GOWEE   STEEET,   W.C. 


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PETROLOGY   FOR    STUDENTS: 


AN   INTRODUCTION    TO   THE   STUDY    OF   ROCKS 
UNDER   THE   MICROSCOPE. 


BY 
ALFRED   BARKER,   M.A.,   F.R.S.,   F.G.S. 

FELLOW   OF   ST   JOHN'S   COLLEGE,    AND 

DEMONSTRATOR   IN    GEOLOGY    (PETROLOGY)    IN 

THE    UNIVERSITY    OF    CAMBRIDGE. 


THIRD     EDITION 


CAMBRIDGE: 

AT   THE   UNIVERSITY    PRESS. 
1902 


First  Edition,  1895. 
Second  Edition,  1897. 
Third  Edition,  1902. 


HFT 


CARTH 

SCIENCES 
.  UBRARY 


PREFACE   TO   THIRD   EDITION. 

r  I  iHE  following  work,  now  offered  in  a  further  revised 
-•-  edition,  has  been  written  to  serve  as  a  guide  to  the 
study  of  rocks  in  thin  slices,  and  is  of  course  assumed  to 
be  supplemented  throughout  by  demonstrations  on  actual 
specimens.  Since  it  is  designed  primarily  for  the  use  of 
English-speaking  students,  examples  are  chosen,  so  far  as 
is  possible,  from  British  and  North  American  rocks ;  and 
a  like  remark  applies  to  the  numerous  references  to  original 
authorities  which  are  inserted  in  foot-no  tes% 

No  systematic  account  is  given  of  the  crystallographic 
and  optical  properties  of  minerals.  This  is  rendered 
unnecessary  by  such  books  as  Iddings'  translation  of 
Rosenbusch's  well-known  work  and  Hatch's  translation 
of  the  same  author's  tables.  In  particular,  I  have 
made  no  explicit  reference  to  the  use  of  convergent 
light. 

In  view  of  the  difficulty  of  representing  rock-sections 
adequately  by  means  of  process-blocks,  I  have  often  cited 

M909018 


VI  PREFACE. 

the  coloured  plates  in  some  standard  works  of  reference, 
to  which  most  students  will  have  access.  The  figures 
given  on  the  following  pages  are  selected  chiefly  to 
illustrate  simple  structural  characters,  and  some  of  them 
are  necessarily  rather  diagrammatic.  A  number  of  new 
figures  have  been  added  for  the  present  edition,  and  a  few 
of  the  old  ones  have  been  withdrawn. 


A.  H. 


ST  JOHN'S  COLLEGE,  CAMBRIDGE. 
October,  1902. 


CONTENTS. 


CHAPTER 

I.  INTRODUCTION     .......  1 

A.  PLUTONIC  ROCKS    .......  23 

II.  GRANITES    ........  28 

III.  SYENITES  (INCLUDING  NEPHELINE-SYENITES)  44 

IV.  DIORITES  ........  5V 

V.  GABBROS  AND  NORITES      ...  .70 

VI.  PERIDOTITES  (INCLUDING  SERPENTINE-ROCKS)  87 

B.  HYP  ABYSSAL  ROCKS      ......  102 

VII.  ACID  INTRUSIVES        ......  103 

VIII.      PORPHYRIES  AND  PORPHYRITES      .        .        .  118 

IX.        DIABASES     ........  130 

X.         LAMPROPHYRES        ......  141 

C.  VOLCANIC  ROCKS         .......  151 

XI.  RHYOLITES      .......  154 

XII.  TRACHYTES  AND  PHONOLITES     .        .        .        .170 

XIII.  ANDESITES       .......  181 

XIV.  BASALTS      ........  194 

XV.  LEUCITE-  AND  NEPHELINE-BASALTS,  ETC.       .  210 

D.  SEDIMENTARY  ROCKS       ......  222 

XVI.  ARENACEOUS  ROCKS        .....  223 

XVII.  ARGILLACEOUS  ROCKS         .....  237 

XVIII.  CALCAREOUS  ROCKS         .....  248 
XIX.      PYROCLASTIC  ROCKS    ......  271 

APPENDIX  TO  SEDIMENTARY  ROCKS  283 


Vlll  CONTENTS. 

CHAPTER  PAGE 

E.     METAMORPHISM 287 

XX.       THERMAL  METAMORPHISM       ....  290 

XXI.  DYNAMIC  METAMORPHISM 315 

XXII.  VARIOUS  CRYSTALLINE  ROCKS        .        .        .  328 

INDEX         .  341 


REFERENCES. 

Berwerth,  Mikroskopische  Structurbilder  der  Massengesteine  (chromo- 
lith.),  Stuttgart,  1895-1900. 

Cohen,  Sammlung  von  Mikrophotographien...von  Mineralien  und 
Gesteinen  (3rd  ed.),  Stuttgart,  1899. 

Rosenbusch-Iddings,  Microscopical  Physiography  of  the  Rock-forming 
Minerals  (with  photographic  plates),  1888. 

Fouque  and  Levy,  Mineralogie  micrographique  (with  atlas  of  coloured 
plates),  1879. 

Teall,  British  Petrography  (with  numerous  coloured  plates),  1888. 

Rosen  busch-Hatch,  Petrographical  Tables. 

Cole,  Studies  in  Microscopical  Science  (coloured  plates),  1882-3. 

Watts,  Guide  — Guide  to  the  Collections  of  Rocks  and  Fossils  be- 
longing to  the  Geological  Survey  of  Ireland^  Dublin,  1895. 


ABBREVIATIONS. 

G.  M.  =  Geological  Magazine. 

M.M.  =  Mineralogical  Magazine. 

Q.J.G.S.  =  Quarterly  Journal  of  Geological  Society. 

A. J.S.  =  American  Journal  of  Science. 


CHAPTER  I. 

INTRODUCTION. 

IN  this  chapter  will  be  included  some  notes  on  the  optical 
properties  of  minerals,  which  may  be  of  use  to  a  novice ;  but 
there  will  be  no  attempt  to  supersede  the  use  of  books  dealing 
systematically  with  the  subject. 

Microscope.  We  shall  assume  the  use  of  a  microscope 
specially  adapted  for  petrological  work,  and  therefore  fitted 
with  polarizing  and  analysing  prisms,  rotating  stage  with 
graduated  circle  and  index,  and  '  cross- wires '  of  spider's  web 
properly  adjusted  in  the  focus  of  the  eye-piece.  The  sub-stage 
mirrors  attached  to  such  instruments  usually  have  a  flat  and 
a  concave  face.  With  day-light  the  flat  face  should  be  used ; 
with  artificial  light  things  should  be  so  arranged  that  the 
mirror,  used  with  the  concave  face,  gives  as  nearly  parallel 
rays  as  possible. 

A  double  nose-piece,  to  carry  two  objectives,  is  very  useful, 
although  it  usually  gives  very  imperfect  centring  for  high 
powers.  The  most  useful  objectives  are  a  1  inch  or  Ij  inch 
and  a  J  inch,  but  for  many  purposes  a  |-  inch  is  also  very 
desirable.  For  minute  objects,  such  as  the  *  crystallites '  in 
glassy  rocks  and  the  fluid-pores  in  crystals,  a  high  power  is 
indispensable,  and  for  very  fine-textured  sedimentary  rocks  an 
immersion-lens  offers  great  advantages. 

A  selenite-plate,  a  quartz- wedge,  and  other  special  pieces  of 
apparatus  will  be  of  use  for  various  purposes.  The  methods 

H.  P.  1 


2          FORM  OF   SECTION  :    MEASUREMENT   OF    ANGLES. 

involving  their  use  may  be  found  in  the  mineralogical  text- 
books ;  where  too  the  student  will  find  guidance  as  to  the 
examination  of  crystal-slices  by  convergent  light. 

Form  of  section  of  a  crystal  and  cleavage-traces. 

A  well-formed  crystal  gives  in  a  thin  slice  a  polygonal  section, 
the  nature  of  which  depends  not  only  upon  the  forms  present 
on  the  crystal,  but  also  on  the  direction  of  the  section  and  on 
its  position  in  the  crystal,  as,  e.g.  whether  it  cuts  through 
the  centre  or  only  truncates  an  edge  or  corner.  Again,  the 
same  shape  of  section  may  be  obtained  from  very  different 
crystals.  Nevertheless,  if  several  crystals  of  one  mineral  are 
present  in  a  rock-slice,  we  can  by  comparison  of  the  several 
polygonal  sections  obtain  a  good  idea  of  the  kind  of  crystal 
which  they  represent.  Further,  if  by  optical  or  other  means 
we  can  determine  approximately  the  crystallographic  direction 
in  which  a  particular  crystal  is  cut,  we  can  usually  ascertain 
what  faces  are  represented  by  the  several  sides  of  the  polygon. 

For  this  purpose  we  may  require  to  measure  the  angle  at 
which  two  sides  meet,  and  this  is  easily  done  with  a  microscope 
provided  with  a  rotating  stage  and  graduated  circle.  Bring 
the  angle  to  the  intersection  of  the  cross-wires,  adjust  one  of 
the  two  sides  to  coincide  with  one  of  the  cross-wires,  and  read 
the  figure  at  the  index  of  the  circle.  Then  rotate  until  the 
other  side  is  brought  to  coincide  with  the  same  cross-wire,  and 
read  the  new  figure.  The  angle  turned  through  is  the  angle 
between  the  two  sides  of  the  section. 

This  angle  is  the  same  as  that  between  the  corresponding 
faces  of  the  crystal  only  provided  the  plane  of  section  cuts 
these  two  faces  perpendicularly.  For  a  section  nearly  perpen- 
dicular to  the  two  faces,  however,  the  error  will  not  be  great. 

In  consequence  of  the  mechanical  forces  which  affect 
rock-masses,  and  also  as  a  result  of  the  process  of  grinding 
rock-slices,  the  minerals  often  become  more  or  less  fractured  or 
even  shattered.  In  a  strictly  homogeneous  substance  the 
resulting  cracks  are  irregular,  but  if  there  be  directions  of 
minimum  cohesion  in  crystals  (cleavage),  the  cracks  will  tend 
to  follow  such  directions,  and  will  appear  in  a  thin  slice  as 
fine  parallel  lines  representing  the  traces  of  the  cleavage-planes 


CLEAVAGE-TRACES  :    TRANSPARENCY.  •) 

on  the  plane  of  section.  The  regularity  and  continuity  of  the 
cracks  give  an  indication  of  the  degree  of  perfection  of  the 
cleavage-structure,  but  it  must  also  be  borne  in  mind  that  a 
cleavage  making  only  a  small  angle  with  the  plane  of  section 
will,  as  a  rule,  not  be  shewn  in  a  slice. 

In  the  case  of  a  mineral  like  augite  or  hornblende,  with 
two  directions  of  perfect  cleavage,  the  angle  which  the  two 
sets  of  planes  make  with  one  another  is,  of  course,  a  specific 
character  of  the  mineral,  or  at  least  characteristic  of  a  group 
of  minerals,  such  as  the  pyroxenes  or  the  amphiboles.  In  a 
slice  perpendicular  to  both  the  cleavages  the  traces  will  shew 
the  true  angle ;  for  any  other  direction  of  section  the  angle 
between  the  cleavage-traces  will  be  different,  but  it  will  not 
vary  greatly  for  slices  nearly  perpendicular  to  both  the 
cleavages,  and  will  often  suffice  for  discrimination,  as  for 
instance  between  the  87°  of  the  pyroxenes  and  the  f>f>J°  of 
the  amphiboles.  In  a  slice  parallel  to  the  intersection  of  the 
two  cleavages  the  two  sets  of  cleavage-traces  reduce  to  one, 
and  a  slice  of  a  mineral  such  as  augite  or  hornblende  which 
exhibits  but  one  set  of  cleavage-traces  may  be  assumed  to  be 
nearly  parallel  to  the  intersection  of  the  cleavages. 

A  mineral  not  possessing  any  good  cleavage  often  shews 
irregular  cracks  in  rock-slices  (e.g.  quartz  and  usually  olivine). 
This  is  especially  the  case  in  brittle  minerals. 

Transparency,  colours,  and  refractive  indices  of 

minerals.  Only  a  few  rock-forming  minerals  remain  opaque 
even  in  the  thinnest  slices  :  such  are  graphite,  magnetite, 
pyrites,  and  pyrrhotite ;  usually  hematite,  ilmenite,  limonite, 
and  kaolin  ;  sometimes  chromite  or  picotite.  These  should 
always  be  examined  in  reflected  light ;  the  lustre  and  colour, 
combined  with  the  forms  of  the  sections  and  sometimes  the 
evidence  of  cleavage,  will  usually  suffice  to  identify  any  of 
these  minerals.  The  great  majority  of  rock-forming  minerals 
become  transparent  in  thin  slices.  Those  which  seen  in  hand- 
specimens  of  rocks  appear  opaque,  are  often  strongly  coloured 
in  slices,  while  those  which  in  hand-specimens  shew  colours 
are  frequently  colourless  in  thin  slices.  In  the  case  of  many 
minerals  these  'absorption-tints'  are  thoroughly  characteristic, 

1—2 


4  REFRACTIVE    INDEX. 

but  still  more  so  are  the  differences  of  colour  (pleochroism)  in 
one  and  the  same  crystal  according  to  the  direction  of  the  slice 
and  the  direction  of  vibration  of  a  polarized  beam  traversing 
it,  as  noticed  below. 

The  colours  ascribed  to  minerals  in  the  following  pages  and 
the  epithet  '  colourless '  apply  to  thin  slices  of  the  minerals. 

Apart  from  colour,  the  aspect  of  a  mineral  as  seen  in  thin 
slices  by  natural  light  varies  greatly  according  to  its  refractive 
index1,  and  it  is  of  great  importance  for  the  student  to  learn 
to  appreciate  at  a  glance  the  effects  due  to  a  high  or  a  low 
refractive  index. 

If  a  thin  slice  of  a  single  crystal  be  mounted  by  itself  in 
some  medium  of  the  same  colour  and  refractive  index  as  the 
crystal,  its  boundaries  and  surface-characters  will  be  invisible, 
while  its  internal  structure  may  be  studied  to  the  best  ad- 
vantage. Quartz  mounted  in  Canada  balsam  (both  colourless 
and  of  very  nearly  the  same  refractive  index)  is  almost  invisible. 
If  olivine,  a  colourless  mineral  of  much  higher  refractive  index, 
be  mounted  in  balsam,  its  boundaries  and  the  slight  roughness 
of  its  polished  surface  will  be  very  apparent2.  In  ordinary 
rock-slices,  mounted  in  balsam,  a  roughened  or  '  shagreened ' 
appearance  may  be  taken  as  the  mark  of  a  mineral  having  a 
refractive  index  considerably  higher  than  that  of  the  medium 
used. 

Again,  a  highly  refringent  mineral .  surrounded  in  the  slice 
by  others  less  highly  refringent  is  seen  to  be  more  strongly 
illuminated  than  these,  and  this  brightness  is  made  more 
conspicuous  by  a  dark  boundary  which  is  deeper  in  proportion 
to  the  difference  in  refractive  index  between  the  mineral  in 
question  and  its  surroundings.  For  these  reasons  a  highly 
refringent  crystal  seems  to  stand  out  in  relief  against  the  rest 
of  the  slice  (fig.  1,  £). 

1  By  this  must  be  understood  its  mean  refractive  index.     A  crystal  of 
any  system  other  than  the  regular  has  in  any  section  two  refractive 
indices,  the  magnitudes  of  which  depend  further  upon  the  direction  of 
the  section ;  but  these  differences  in  any  one  mineral  are  usually  small  as 
compared  with  the  differences  between  the  mean  indices  in  different 
minerals. 

2  Cohen  (3),  pi.  XTATTII,  compare  figs.  1  and  2, 


REFRACTIVE    INDEX. 


Such  considerations  must  be  borne  in  mind  in  examining 
the  minute  inclusions  in  which  many  crystals  abound.  These 
inclusions  may  be  of  gas,  of  liquid  (usually  with  a  gaseous 
bubble),  of  glass,  or  a  crystal  of  some  other  mineral ;  and  these 
may  be  distinguished  by  observing  that  the  depth  of  the  dark 
border  depends  upon  the  difference  in  refractive  index  between 


FIG.  1.  VARIOUS  MICROSCOPIC  INCLUSIONS,  HIGHLY  MAGNIFIED. 
a.  Gas-pores ;  in  obsidian.  b.  Fluid-pores  with  bubbles ;  in 
quartz.  c.  Fluid-pore  with  bubble  and  cube  of  salt;  in  quartz. 
d.  Fluid-cavity  in  form  of  'negative  crystal,'  containing  two  fluids  and 
bubble  ;  in  quartz.  e.  Fluid-cavities  in  form  of  '  negative  crystals,' 
with  bubbles ;  in  quartz.  /.  Glass-inclusions  in  form  of  '  negative 
crystals,'  with  bubbles;  in  quartz.  g.  Schiller-inclusions  consisting 
of  three  sets  of  flat  '  negative  crystals '  filled  with  opaque  iron-oxide  ;  in 
felspar.  //..  Hchiller-inclusions  consisting  of  '  negative  crystals '  partly 
occupied  by  a  dendritic  growth  of  iron-oxide ;  in  olivine.  k.  Zircon- 
crystal  enclosed  in  quartz  and  itself  enclosing  an  apatite- needle. 

the  enclosing  and  the  enclosed  substance1  (fig.  1).  The  most 
strongly  marked  border  is  seen  when  a  gaseous  is  enclosed  by 
a  solid  substance  (a).  A  liquid-inclusion  in  a  crystal  has  a 

1  For  figures  of  various  inclusions  in  crystals  see  Cohen  (3),  pi.  vm — 
xin ;  Rosenbusch-Iddings,  pi.  vi,  vn ;  Sorby,  Q.  J.  G.  S.  (1858)  xiv, 
pi.  xvi — xix ;  Ward,  ibid.  (1875)  xxxi,  pi.  xxx. 


6  TABLE   OF   REFRACTIVE    INDICES. 

less  marked  boundary,  but  a  bubble  of  vapour  in  the  liquid  is 
strongly  accentuated  (b — e).  A  glass-inclusion  is  still  less 
strongly  marked  off  from  its  enclosing  crystal,  while  a  gas- 
bubble  contained  in  it  shews  a  very  deep  black  border  (/). 

When  two  minerals  (or  a  mineral  and  Canada  balsam)  are 
in  contact  with  one  another  in  a  thin  slice  in  such  a  position 
that  their  surface  of  junction  is  cut  approximately  at  right 
angles  by  the  plane  of  section,  it  is  easy  to  determine  which 
of  the  two  has  the  higher  refractive  index.  For  this  purpose 
the  illumination  should  be  limited  by  a  diaphragm  placed 
below  the  stage,  and  a  high-power  objective  focused  upon  the 
line  of  junction  at  the  upper  surface  of  the  slice.  This  line  is 
then  seen  to  be  bordered  by  a  narrow  bright  band  on  the  side 
of  the  more  highly  refringent  mineral  and  a  narrow  dark  band 
on  the  other  side.  If  the  objective  be  depressed  until  the 
lower  surface  of  the  slice  is  in  focus,  these  appearances  are 
reversed. 

The  refractive  indices  of  the  several  rock-forming  minerals 
may  be  found  in  the  tables  or  books  of  reference,  but  the 
student  will  find  it  useful  to  carry  in  his  mind  such  a  list  as 
that  given  below. 

Refractive  indices  of  the  common  rock-forming  minerals. 

Very  low  (1*43 — 1'51) :  tridymite,  sodalite,  analcime  and 
most  other  zeolites,  (volcanic  glasses),  leucite. 

Low  (1'52 — 1'63)  :  felspars,  nepheline,  quartz,  (Canada  balsam), 
micas,  calcite,  dolomite,  wollastonite,  actinolite,  melilite. 

Moderate  (1/63 — 1'645)  :  apatite,  tourmaline,  andalusite,  horn- 
blende. 

High  (1/68 — 1'8)  :  olivine,  sillimanite,  pyroxenes,  zoisite, 
idocrase,  epidote,  garnets. 

Very  high  (1'9 — 1'95)  :  sphene,  zircon. 

Extremely  high  (2'0 — 2' 7)  :  chromite,  rutile. 

Extinction  between  crossed  nicols.  When  the 
polarizing  and  analysing  Nicol's  prisms  are  used  together, 
with  their  planes  of  vibration  at  right  angles  to  one  another 


AXES   OF   EXTINCTION.  7 

('crossed  nicols')1,  if  no  object  be  interposed,  there  is  total 
darkness  ('  extinction '),  and  the  same  is  the  case  when  a  slice 
of  any  vitreous  substance,  such  as  obsidian,  is  placed  on  the 
stage.  If,  however,  a  slice  of  a  crystal  of  any  system  other 
than  the  regular  is  interposed,  there  is  in  general  more  or  less 
illumination  transmitted,  and  often  bright  colours.  On  ro- 
tating the  stage2  carrying  the  object,  it  is  found  that  extinction 
takes  place  for  four  positions  during  a  complete  rotation,  these 
being  at  intervals  of  a  right  angle.  In  other  words,  there  are 
two  axes  of  extinction  at  right  angles  to  one  another  and  the 
slice  remains  dark  only  while  these  axes  are  parallel  to  the 
planes  of  vibration  of  the  nicols,  which  are  indicated  by  the 
cross-wires  in  the  eye-piece.  If  we  rotate  the  slice  into  a 
position  of  extinction  and  then  remove  the  nicols,  the  cross- 
wires  will  mark  the  axes  of  extinction  in  the  crystal-slice. 

Without  attempting  to  deal  fully  with  this  branch  of 
physical  optics3,  we  may  remark  that  all  the  optical  properties 
of  a  crystal  are  related  to  three  straight  lines  conceived  as 
drawn  within  the  crystal  at  right  angles  to  one  another  (the 
axes  of  optic  elasticity)  and  to  a  certain  ellipsoid  having  these 
three  straight  lines  for  axes  (the  ellipsoid  of  optic  elasticity). 
The  positions  of  the  three  axes  may  vary  in  different  minerals, 
but  they  must  always  conform  to  the  symmetry  proper  to  the 
system,  and  the  same  is  true  of  the  relative  lengths  of  the 
axes  of  the  ellipsoid.  The  plane  of  section  of  any  slice  cuts 
the  ellipsoid  in  an  ellipse,  the  form  and  position  of  which 
depend  upon  the  direction  of  the  section  (ellipse  of  optic  elas- 
ticity), and  the  axes  of  extinction  are  the  axes  of  this  ellipse. 

In  certain  cases  the  ellipse  of  optic  elasticity  may  be  a 

1  In  using  the  two  Nicol's  prisms,  it  should  always  be  ascertained  that 
they  are  crossed.     For  this   purpose   the   rotating  prisms   are   usually 
provided  with  catches  in  the  proper  positions,  but  the  true  test  is  total 
darkness  when  no  object  is  interposed. 

2  In  some  microscopes,  such  as  that  devised  by  Mr  A.  Dick,  the  stage 
is  fixed,  and  the  two  nicols  rotate,  retaining  their  relative  position,  an 
arrangement  with  several  advantages.     We  shall  assume  for  distinctness 
that  the  stage  is  made  to  rotate,  as  in  the  most  usual  models. 

3  The  student  is  referred  for  this  to  such   a   book  as   Kosenbusch 
(transl.  Iddings),  Microscopical  Physiography  of  the  Rock-makiny  Minerals 
(1888),  London. 


8  STRAIGHT   AND   OBLIQUE    EXTINCTION. 

circle.  For  this  any  diameter  is  an  axis,  and  accordingly  we 
find  that  such  a  slice  gives  extinction  throughout  the  complete 
rotation.  In  crystals  of  the  triclinic,  monoclinic,  and  rhombic 
systems  there  are  two  directions  of  section  which  give  this 
result.  They  are  perpendicular  respectively  to  two  straight 
lines  in  the  crystal  (the  optic  a,res),  which  lie  in  the  plane  of 
two  of  the  axes  of  optic  elasticity,  and  are  symmetrically 
disposed  towards  them.  In  crystals  of  the  tetragonal  and 
rhombohedral  systems  the  two  optic  axes  coincide  with  one 
another  and  with  the  unique  crystallographic  axis,  and  only 
slices  perpendicular  to  this  give  total  darkness.  In  the 
regular  system,  the  ellipsoid  being  a  sphere,  the  ellipse  is 
always  a  circle,  and  all  slices  give  total  darkness  between 
crossed  nicols. 

Crystals  of  the  regular  system  are  spoken  of  as  singly 
refracting  or  optically  isotropic,  and  their  optical  properties1 
are  similar  to  those  of  a  glassy  or  colloid  substance.  Crystals 
of  the  other  systems  are  doubly  refracting  or  birefringent,  and 
they  are  divided  into  uniaxial  or  biaxial  according  as  they 
have  one  or  two  optic  axes. 

It  is  evident  that  the  chance  of  a  slice  cut  at  random  from 
a  birefringent  crystal  being  perpendicular  to  an  optic  axis  is 
very  small.  If  more  than  one  crystal  of  a  given  mineral  be 
present  in  a  rock-slice,  and  all  remain  perfectly  dark  between 
crossed  nicols  throughout  a  rotation,  it  is  a  safe  conclusion 
that  the  mineral  is  a  singly  refracting  one. 

Straight  and  oblique  extinction.  By  bearing  in 
mind  that  the  ellipsoid  of  optic  elasticity,  and  consequently 
all  the  optical  properties  of  a  crystal,  must  conform  to  the 
laws  of  symmetry  proper  to  the  crystal-system  of  the  mineral, 
we  can  foresee  all  the  important  points  as  regards  the  position 
of  the  axes  of  extinction  in  crystals  of  the  different  systems 
cut  in  various  directions.  For  instance,  a  longitudinal  section 
of  a  prism  of  apatite  (a  hexagonal  mineral)  will  extinguish 
when  its  length  is  parallel  to  either  of  the  cross-wires  :  this  is 
straight  extinction.  A  longitudinal  section  of  a  prism  of 

1  That  is,  such  of  them  as  we  are  here  concerned  with. 


MEASUREMENT    OF    EXTINCTION-ANGLES. 

albite  (a  triclinic  mineral)  will,  on  the  other  hand,  have  axes 
of  extinction  inclined  at  some  angle  to  its  length  :  this  is 
oblique  extinction.  It  is  to  be  noticed  that  these  terms  have 
no  meaning  unless  it  is  stated  or  clearly  understood  from  what 
direction  in  the  crystal  the  obliquity  is  reckoned.  In  these 
examples  we  reckoned  with  reference  to  one  of  the  crystallo- 
graphic axes  defined  by  the  traces  of  known  crystal-faces. 
Another  character  often  utilised  is  the  cleavage.  Thus  in  a 
monoclinic  mineral  with  prismatic  cleavages,  such  as  horn- 
blende, we  select  a  crystal  so  cut  that  the  two  cleavages  give 
only  one  set  of  parallel  traces.  These  traces  are  then  parallel 
to  one  of  the  crystallographic  axes  (the  vertical  axis),  and  we 
examine  the  position  of  extinction  with  reference  to  this. 
First  we  bring  the  cleavage-traces  parallel  to  one  of  the 
cross-wires,  removing  if  necessary  for  this  purpose  one  or  both 
of  the  nicols,  and  note  the  figure  indicated  on  the  graduated 
circle.  Then,  with  crossed  nicols,  we  rotate  until  the  crystal 
becomes  dark,  and  again  note  the  figure.  The  angle  through 
which  we  have  turned  is  the  extinction-angle.  Observe  that 
if  a  rotation  through,  say,  15°  in  one  direction  gives  extinction, 
a  rotation  through  75°  in  the  opposite  direction  would  have 
given  the  same.  For  most  purposes  we  do  not  need  to 
distinguish  between  the  two  directions  of  rotation,  but  take 
merely  the  smaller  of  the  two  angles. 

To  obtain  a  measurement  of  use  in  identifying  a  mineral 
we  require  more  than  the  above.  Slices  of  a  crystal  of 
hornblende  cut  in  various  directions  along  the  vertical  axis 
will  give  different  extinction-angles,  from  zero  (straight 
extinction)  in  a  section  parallel  to  the  orthopinacoid  to  a 
maximum  value  in  a  certain  other  section.  This  maximum 
extinction-angle  is  a  character  of  specific  value,  being  the 
angle  between  the  vertical  crystallographic  axis  and  the 
nearest  axis  of  optic  elasticity.  We  may  determine  it  with 
sufficient  accuracy  for  most  purposes  by  noting  the  ex- 
tinction-angles in  two  or  three  vertical  sections  of  the  same 
mineral  in  a  rock-slice  and  taking  the  largest  value  obtained1. 

1  On  the  relation  between  this  maximum  extinction-angle  and  the 
extinction-angle  measured  in  a  cleavage-flake  of  hornblende  or  augite, 
see  M.  M.  (1893)  x,  239,  240 ;  and  Daly,  Proc.  Amer.  Acad.  Arts  and  Sci. 
(1899)  xxxiv,  311—323. 


10  CRYSTALLOGRAPHIC  SYSTEMS  :    TWINNING. 

By  attention  to  the  following  points  it  is  in  most  cases 
possible  to  refer  to  its  crystal-system  an  unknown  mineral  of 
which  several  sections  are  presented  in  a  rock-slice : 
Regular  system  :  singly  refracting  ;  all  slices  extinguish  com- 
pletely between  crossed  nicols,  as  in  glassy  substances. 
Tetragonal  and  Rhombohedral  (including  Hexagonal) :  bire- 
fringent  and  uniaxial ;  straight  extinction  for  longitudinal 
sections   of  crystals   with  prismatic   habit    and   for  any 
sections  of  crystals  with  tabular  habit.     The  two  systems 
cannot  be  distinguished  from  one  another  by  optical  tests, 
but   in  cross-sections   of  prisms   the    crystal    outline   or 
cleavages  will  usually  suffice  to  discriminate. 
Rhombic  (this  and  the  remaining  systems  birefringent  and 
biaxial)  :  straight  extinction  for  longitudinal  sections  of 
crystals  with  prismatic  habit ;  sections  perpendicular  to  the 
vertical  axis  have  axes  of  extinction  parallel  to  pinacoidal 
faces  or  cleavages  and  bisecting  the  angles  between  the 
traces  of  prism-faces  or  prismatic  cleavages.     A  section 
nearly  parallel  to  the  vertical  axis  will  give  nearly  straight 
extinction,  except  in  minerals  which  have  a  wide  angle 
between  the  optic  axes. 

Monoclinic :  two  important  types  may  be  noticed  according  as 
the  intersection  of  the  chief  cleavages  (and  direction  of 
elongation  of  the  crystals)  lies  in  or  perpendicular  to  the 
plane  of  symmetry.  In  the  former  case  longitudinal 
sections  may  give  any  extinction-angle  from  zero  up  to  a 
maximum  value  characteristic  of  the  species  or  variety :  in 
the  latter  (e.g.  epidote  and  wollastonite)  longitudinal 
sections  give  straight  extinction.  The  former  case  is  the 
more  frequent. 

Triclinic  :  no  sections  give  systematically  straight  extinction. 
Twinnhu/.  The  existence  of  twinning  in  a  slice  of  a 
crystal  is,  in  general,  instantly  revealed  by  an  examination 
of  the  slice  between  crossed  nicols,  since  the  two  individuals 
of  the  twin  shew  different  interference-tints  and  extinguish  in 
different  positions1.  When  twin-plane  and  face  of  association 

1  The  only  exceptions  (apart  from  opaque,  crystals)  are  in  minerals, 
like  the  spinels,  optically  isotropic,  and  in  cases  in  which  the  law  of 
twinning  is  such  that  the  directions  of  the  axes  of  optical  elasticity  are 
not  altered  (e.g.  quartz). 


EXTINCTION- ANGLES   IN    FELSPARS.  11 

coincide — the  most  common  case — a  slice  perpendicular  to 
the  twin-plane  will  give  in  the  two  individuals  of  the  twin 
extinction-angles  which,  reckoned  from  the  line  of  junction, 
are  equal  but  in  opposite  directions.  Conversely,  a  crystal 
which  gives  equal  but  opposite  extinction -angles  may  be 
assumed  to  be  cut  very  nearly  perpendicularly  to  the  twin- 
plane.  If  the  plane  of  section  cut  the  twin-plane  of  a  crystal 
at  a  very  small  angle,  the  two  individuals  of  the  twin  will 
overlap  for  a  sensible  width,  and  we  shall  see  between  the  two 
a  narrow  band  which  does  not  behave  optically  with  either. 
When  repeated  twinning  occurs,  as  in  felspars  with  albite 
lamellation,  the  lamella  divide,  as  regards  optical  behaviour, 
into  two  sets  arranged  alternately. 

Extinction-angles  in  felspars.  The  discrimination 
of  the  several  felspars  by  means  of  their  extinction-angles 
measured  on  cleavage-flakes,  as  perfected  by  Schuster,  is  a 
method  of  great  precision,  but  is  not  applicable  to  crystals  in 
rock-slices.  For  these  the  method  advocated  by  Michel  Levy 
and  others  will  often  be  found  useful.  There  are  two  cases  in 
which  it  is  readily  applied. 

(i)  For  crystals  with  albite-lamellation  : — Select  sections 
cut  approximately  perpendicular  to  the  lamella).  These  are 
known  by  the  extinction-angles  in  the  two  alternating  sets  of 
lamellae,  reckoned  from  the  twin-line,  being  in  opposite 
directions  and  nearly  equal ;  also  by  the  illumination  of  the 
two  sets  of  lamella)  being  not  very  different  when  the  twin-line 
is  parallel  to  a  cross-wire.  Measure  the  angles  in  question  in 
three  or  four  crystals  so  selected,  and  take  the  greatest  value 
found.  This  will  be  very  nearly  the  maximum  angle  for  all 
such  sections,  which  is  a  specific  constant  for  each  kind  of 
felspar,  as  indicated  for  certain  types  in  the  annexed  diagram. 
The  values  for  types  not  given  in  the  diagram  may  be  judged 
with  sufficient  accuracy  by  interpolation,  since  the  maximum 
extinction-angle  changes  steadily  from  one  end  of  the  series  to 
the  other.  It  will  be  noticed,  however,  that  in  certain  cases 
different  kinds  of  felspar  (viz.  those  placed  on  the  same 
vertical  lines  in  the  diagram,  fig.  2)  give  equal  angles,  and  in 
this  connection  two  remarks  are  to  be  made. 


12 


EXTINCTION-ANGLES   IN    FELSPARS. 


OLIG. 


ACID 


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ALB. 


OLIG. 


ME  D.I 


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LAB. 

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ANO. 


BYT. 


FIG.  2.     MAXIMUM  EXTINCTION-ANGLES  OF  PLAGIOCLASE  FELSPABS  IN 
SECTIONS  AT  EIGHT  ANGLES  TO  THE  ALBITE-LAMELL^. 


FIG.  3.     MAXIMUM  EXTINCTION- ANGLES  OF  PLAGIOCLASE  FELSPARS  IN 
LONGITUDINAL  SECTIONS  OF  MICKOLITE8. 


EXTINCTION-ANGLES   IN   LAMELLATED   FELSPARS.       13 

(a)  A  slice  of  a  crystal  has  two  directions  of  extinction, 
at  right  angles  to  one  another.     Hitherto  we  have  taken  the 
angle  to  the  nearest  direction  of  extinction,  but  the  diagram 
shews   that  for  angles   of  37°   or   more   this  introduces  an 
ambiguity.     It  is  then  necessary  to  distinguish  between  the 
two  directions  (by  means  of  the  quartz-wedge  or  some  other 
contrivance)  and  to  select  that  one  which  corresponds  with  the 
least  axis  of  the  ellipse  of  elasticity  (indicated  by  an  arrow- 
head in  the  diagram).     In  this  way  anorthite  and  bytownite 
are   discriminated   from    the    medium    labradorites.      Other 
criteria  may  sometimes  be  used,  e.g.  the  stronger  birefringence 
of  anorthite,  as  pointed  out  below1. 

(b)  The  signs  +  and  —  denote  angles  measured  in  opposite 
directions  crystallographically.     Unless  other  means  of  discri- 
mination can  be  made  use  of,  we  have  usually  no  way  of 
distinguishing  the  two  directions,  and  there  is  consequently 
an  ambiguity  between  albite  and  the  more  basic  oligoclases 
(with  oligoclase-andesine).     Since  the  latter  have  about  the 
same  refractive  index  as  quartz  and  Canada  balsam,  while  the 
index   for   albite   is   distinctly   lower,    a   discrimination  may 
sometimes   be  made   by   rough  observations  of  comparative 
refringence. 

Summarily,  we  have  the  following  characteristic  angles  for 
different  felspars  :— 

0°  to    5°,  oligoclase,  the  more  acid  types. 

6°  to  16°,  albite  and  the  more  basic  oligoclases  (with 

oligoclase-andesine). 

16°  to  22°,  andesines. 

27°  to  45°,  labradorites. 

45°  to  50°,  bytownites. 

50°  and  above,  anorthites. 

(ii)  ^  For  imcrolites,  assumed  to  have  their  length  parallel 
to  the  intersection  of  the  two  principal  cleavages  : — Here  we 
measure  extinction-angles  from  the  long  axis  of  the  microlites, 

1  Another  point  worthy  of  notice  is  the  frequency  with  which  certain 
angles  (less  than  the  maximum)  occur  in  a  number  of  sections  perpendi- 
cular to  the  albite-lamellre.  For  anorthite  the  favorite  angles  are  32° 
and  41°,  for  medium  labradorite  21°  and  36°. 


14         EXTINCTION-ANGLES   IN   FELSPAfc-MICROLITES. 

and  select  the  highest  angle  obtained  by  measurements  on 
several  microlites.  The  characteristic  maxima  for  certain 
varieties  of  plagioclase  are  given  in  the  annexed  diagram 
(fig.  3),  and  the  values  for  intermediate  varieties  can  be 
interpolated.  As  before,  there  are  two  points  to  be  noted. 

(a)  If  the  angle  of  extinction  as  measured  is  26°  or  more, 
we  must  discriminate    by    the    quartz-wedge    or    otherwise 
between  the  two  directions  of  extinction. 

(b)  If  the  angle  is  20°  or  less,  an  ambiguity  occurs  which 
cannot  be  removed  by  this  method ;  viz.  between  albite  and 
andesine  or  andesine-labradorite  and  between  acid  oligoclase 
and   oligoclase-andesine.     There   is   thus    more    unavoidable 
ambiguity   in  this  case   than   in   that   of  albite-lamellre,   as 
appears  from  the  following  values  for  different  felspars. 

0°  to     7°,  oligoclase  with  oligoclase-andesine. 
8°  to  10°,  albite-oligoclase  and  andesine. 

10°  to  20°,  albite    and    andesine-labradorite    with    acid 
labradorite. 

30°  to  42°,  labradorite,  medium  to  basic. 
49°  to  56°,  bytownites. 
58°  to  64°,  anorthites. 

Becker1  has  suggested  another  test  applicable  to  microlites, 
which  may  very  conveniently  be  used  to  supplement  the  above ; 
since,  although  it  is  of  little  use  for  the  more  basic  varieties, 
it  affords  a  useful  criterion  for  distinguishing  the  oligoclases, 
andesines,  etc.  Instead  of  longitudinal  sections,  perpendicular 
cross-sections  of  the  microlites  are  selected.  These  are  small, 
nearly  square,  and  sharply  denned.  The  extinction-angles 
vary  from  —13°  for  pure  albite  to  42  J°  for  anorthite,  and  from 
Becker's  figures  we  may  deduce  the  following  approximate 
values  : — 


i  ISth  Ann.  Rep.  U.S.  Geol.  Sur.  (1898)  part  in,  32—34,  and  A.  J.  8. 
(1898)  v,  349 — 354,  pi.  nr.  For  a  more  general  account  of  the  modern 
optical  methods  of  discriminating  the  felspars  see  Winchell,  Amer.  Geol. 
(1898)  xxi,  12 — 48,  pi.  n — vm. 


ZONARY   BANDING   IN   FELSPARS.  15 

0°  to      4°,  oligoclase,  acid. 

4°  to      7°,  oligoclase,  medium,  and  albite-oligoclase. 
7°  to    13°,  oligoclase,  basic,  and  albite. 
18°  to    22°,  andesine. 
26j°  to    38°,  labradorite,  acid  to  medium. 
38°  to  42j°,  medium  labradorite  to  anorthite. 

If  the  sections  selected  for  measurement  be  as  much  as  10° 
from  the  true  perpendicular  cross-section,  the  resulting  error  is 
only  about  1|°  to  2j°  in  the  more  acid  half  of  the  plagioclase 
series,  and  therefore  does  not  vitiate  the  conclusion. 

Zonary  banding  in  felspars.  In  many  rocks  the  felspars 
shew  between  crossed  nicols  concentric  zones  roughly  parallel 
to  the  boundary  of  the  crystal,  the  successive  zones  extin- 
guishing in  different  positions.  (If  there  be  albite-lamellation, 
we  confine  our  attention  to  one  of  the  two  sets  of  lamella).) 
This  difference  in  optical  behaviour  among  the  successive  layers 
which  build  up  the  crystal  may  arise  in  two  ways  :  firstly, 
from  the  successive  zones  being  of  different  kinds  of  felspar- 
substance  ;  or,  secondly,  from  ultra-microscopic  twinning 
affecting  in  various  degrees  the  different  layers  of  a  crystal 
chemically  homogeneous.  This  has  been  pointed  out  by 
Michel  Ldvy,  and  he  gives  a  test  which  will  resolve  all  except 
certain  rare  cases.  It  will  be  found,  on  rotating  the  slice 
between  crossed  nicols,  that  there  are  certain  positions  in 
which  the  albite-lamellse  disappear.  If  simultaneously  with 
this  the  zonary  banding  disappears  also,  so  that  the  whole 
crystal1  is  uniformly  illuminated,  the  appearances  can  be 
explained  by  ultra-microscopic  twinning  alone  :  if  this  is  not 
the  case,  the  zonary  banding  may  be  ascribed  to  the  successive 
layers  of  felspar-substance  in  each  crystal  differing  in  chemical 
composition.  When  this  occurs,  the  rule  generally  holds  that 
the  layers  or  zones  become  progressively  more  acid  from  the 
centre  to  the  margin. 

Interference-tints.  We  have  remarked  that  a  thin 
slice  of  a  doubly  refracting  crystal,  examined  between  crossed 
nicols,  is  in  general  not  dark  except  when  placed  in  certain 

1  Or  if  there  be  Carlsbad  twinning  also,  the  whole  of  one  individual 
of  the  Carlsbad  twin. 


16  INTERFERENCE-TINTS. 

definite  positions.  In  any  other  position  it  does  not  completely 
extinguish  the  light,  but  its  effect,  in  conjunction  with  the 
nicpls,  is  partially  to  suppress  the  several  components  of  the 
white  light  in  different  degrees,  so  that  in  the  emergent  beam 
these  components  are  no  longer  in  the  proportions  to  give  white 
light.  In  this  way  arise  polarization-tints  or  interference-tints. 
These  belong  to  a  definite  scale,  known  as  Newton's  scale,  on 
which  the  several  tints  (though  graduating  into  one  another) 
are  distinguished  by  names  and  divided  into  several  'orders.' 
The  student  should  learn  the  succession  of  these  tints,  in  the 
first  place  from  the  coloured  plates  accompanying  some  mineral- 
ogical  works1,  but  ultimately  from  the  minerals  themselves. 
The  precise  position  in  the  scale  of  a  given  tint  observed 
between  crossed  nicols  can  be  fixed  by  means  of  a  quartz- 
wedge  or  other  contrivance  for  '  compensating '  or  neutralising 
the  birefringence  of  the  slice ;  but  for  ordinary  purposes,  at 
least  with  colourless  or  nearly  colourless  minerals,  the  inter- 
ference-tint can  be  judged  by  eye  with  sufficient  accuracy. 
The  most  brilliant  colours  are  those  of  the  second  order  and 
at  the  top  of  the  first ;  the  lowest  colours  of  the  first  order  are 
dull  greys;  while  in  the  third  and  fourth  orders  the  tints 
become  brighter  but  paler,  ultimately  approximating  to  white. 

The  interference-tints  given  by  a  crystal-section  depend 
(i)  on  the  birefringence  of  the  mineral,  which  is  a  specific 
character ;  (ii)  on  the  direction  of  the  section  relatively  to  the 
ellipsoid  of  optic  elasticity,  the  tint  being  highest  for  a  section 
parallel  to  the  greatest  and  least  axes  of  the  ellipsoid  ;  (iii)  on 
the  thickness  of  the  slice.  These  last  two  are  disturbing 
factors,  which  must  be  eliminated  before  we  can  use  the  inter- 
ference-tints as  an  index  of  the  birefringence  of  the  crystal, 
and  so  as  a  useful  criterion  in  identifying  the  mineral. 

The  fact  that  the  interference-tints  depend  in  part  on  the 
direction  of  the  section  through  the  crystal  will  rarely  be 
found  to  give  rise  to  any  difficulty  in  estimating  roughly  the 
birefringence  of  the  mineral.  If  two  or  three  crystals  of  the 
same  mineral  are  contained  in  a  rock-slice,  it  is  sufficient  to 

1  Michel  L6vy  and  Lacroix,  Les  Mineraux  des  Roches :  Rosenbusch 
(transl.  Iddings),  Microscopical  Physiography  of  the  Rock-making 
Minerals. 


TABLE   OF   BIREFRINGENCE   AND   INTERFERENCE-TINTS.     17 

have  regard  to  that  one  which  gives  the  highest  interference- 
tints.  Even  a  single  crystal  will  in  the  majority  of  cases  give 
tints  not  so  far  below  those  proper  to  the  mineral  as  to  occasion 
error,  but  the  possibility  of  the  section  having  an  unlucky 
direction  must  be  borne  in  mind. 

Rock-slices  prepared  by  a  skilful  operator  are  in  most  cases 
so  nearly  constant  in  thickness  that  variations  in  this  respect 
may  be  left  out  of  consideration.  Any  important  difference  is 
at  once  detected  by  well-known  minerals  giving  unusual  inter- 
ference-tints. Thus  if  quartz  or  orthoclase  give  the  yellow  of 
the  first  order,  the  slice  is  rather  a  thick  one ;  if  they  give 
orange  or  red,  the  slice  is  considerably  thicker  than  the  average 
of  good  preparations.  Knowing  this,  we  can  make  allowance 
for  it  in  estimating  the  birefringence  of  some  doubtful  mineral 
in  the  same  slice.  Such  allowance  can  be  roughly  judged,  or 
it  can  be  made  with  considerable  precision  by  means  of  the 
large  coloured  plate  of  Michel  LeVy  and  Lacroix1. 

The  actual  birefringence  (numerically  expressed)  of  the 
several  rock-forming  minerals,  and  the  interference-tints  which 
they  afford  in  slices  of  ordinary  thickness,  are  given  in 
numerous  books  and  tables.  For  rough  purposes  the  student 
will  find  it  useful  to  remember  about  as  much  as  is  contained 
in  the  following  table. 

Birefringence  and  interference-tints  of  the  commoner  rock- 
forming  minerals.     (The  colours  given  are  for  slices  "001  inch 
in  thickness.) 
I  rery  weak  (giving  steel-grey  tints) :  leu  cite,  apatite,  nepheline, 

melilite. 
Weak  (giving  blue-grey  to  white  of  first  order) :  zoisite,  micro- 

cline,  orthoclase,  albite,  oligoclase,  andesine,  labradorite, 

quartz,  bytownite,  enstatite. 
Moderate  (giving  white,  yellow,  or  orange  of  first  order) :  anda- 

lusite,  chlorite,  anorthite,  hypersthene. 
Strong  (giving  red  of  first  order  to  violet  and  blue  of  second) : 

tourmaline,  augite  and  diallage,  common  hornblende  and 

actinolite. 

1  This  plate  can  be  purchased  separately  and  mounted  as  a  wall- 
diagram.  On  the  method  of  using  it  see  Pirsson  and  Robinson,  A.  J.  S. 
(1900)  x,  260—265. 

H.  P.  2 


18  PLEOCtiROlSM. 

Very  strong  (giving  green,  yellow,  or  orange  of  second  order)  : 
olivine,  epidote,  talc,  biotite,  muscovite. 

Extremely  strong  (giving  the  pale  colours  of  the  third  and 
fourth  orders  to  almost  pure  white)  :  zircon,  hornblende 
rich  in  iron,  sphene,  calcite  and  dolomite,  rutile. 

Note  that  in  minerals  with  strong  absorption,  such  as  the 
deep-coloured  micas  and  hornblendes,  the  interference-colours 
are  more  or  less  masked  by  those  due  to  absorption. 

Pleochroism.  A  character  often  useful  in  identifying 
minerals  is  pleochroism,  the  property  of  giving  different  ab- 
sorption-tints for  different  directions  of  vibration  of  the  light 
within  the  crystal.  To  observe  this  property,  we  use  the  lower 
nicol  only,  and  rotate  either  it  or  the  stage.  The  direction  of 
vibration  is  that  of  the  shorter  diagonal  of  the  nicol. 

It  is  necessary  not  only  to  observe  the  changes  of  colour, 
if  any,  but  also  to  note  their  relation  to  directions  of  vibration 
within  the  crystal.  For  example,  elongated  sections  of  biotite 
and  hornblende,  tourmaline  and  sphene,  may  be  found  to 
change  from  a  deeper  to  a  paler  tint  of  brown  on  rotation ;  but 
while  in  the  first  pair  of  minerals  the  direction  of  vibration 
most  nearly  coincident  with  the  long  axis  of  the  section  gives 
the  deeper  tone,  in  the  second  pair  it  gives  the  paler. 

To  be  more  precise,  we  wish  to  know,  for  a  specification  of 
the  pleochroism  of  a  given  mineral,  the  absorption-tints  for 
vibrations  in  three  definite  directions  within  the  crystal — those 
of  the  three  axes  of  optical  elasticity.  Taking  a  given  mineral, 
say  a  hornblende,  of  which  a  number  of  crystals  occur  in  our 
slice,  we  may  proceed  as  follows.  Select  a  crystal  shewing 
only  one  set  of  cleavage-traces  and  giving  the  maximum 
extinction-angle  :  this  section  will  be  approximately  parallel 
to  the  plane  of  symmetry,  which  contains  two  of  the  required 
axes.  These  axes  are  the  axes  of  extinction  for  the  section, 
and  their  positions  are  thus  easily  found.  The  one  nearest  to 
the  cleavage-traces  is  the  y-axis,  the  other  the  a-axis.  Bring 
the  y-axis  to  coincide  in  direction  with  the  shorter  diagonal  of 
the  nicol,  adjusting  the  position  by  obtaining  extinction,  and 
then  removing  the  upper  nicol.  Observe  the  colour  :  then  do 
the  same  for  the  a-axis.  For  the  remaining  /?-axis  we  must 


EXAMINATION   OF   A   ROCK-SLICE.  19 

use  another  crystal.  We  may  choose  one  shewing  only  a 
single  set  of  cleavage-traces  and  giving  straight  extinction  : 
the  /?-axis  is  perpendicular  to  the  cleavage-traces.  Or  we  may 
choose  a  section  shewing  two  sets  of  cleavage-traces  intersecting 
at  a  good  angle  and  extinguishing  along  the  bisectors  of  the 
angles  between  the  cleavage-traces  :  the  /2-axis  is  the  bisector 
of  the  acute  angle. 

Minerals  of  the  rhombohedral  and  tetragonal  systems  can 
have  only  two  distinct  absorption-tints  (dichroism),  one  for 
vibrations  parallel  to  the  longitudinal  axis  (extraordinary  ray), 
the  other  for  vibrations  in  any  direction  perpendicular  to  it 
(ordinary  ray).  In  the  regular  system  the  absorption-colours 
are  independent  of  direction. 

In  consequence  of  pleochroisrn  the  absorption-tints  of  a 
mineral  vary  in  differently  cut  crystals  seen  in  natural  light, 
but  the  precise  nature  of  the  pleochroism  can  be  investigated 
only  with  a  polarized  beam. 

Examination  of  a  rock-slice.  In  studying  a  rock- 
slice  it  is  always  well  to  proceed  methodically.  A  low  power 
should  first  be  used :  any  object  which  it  is  desirable  to  examine 
under  a  higher  magnification  should  be  brought  to  the  centre 
of  the  field  before  the  objective  is  changed  for  a  higher  power. 
The  slice  should  always  be  observed  first  in  natural  light :  by 
their  outline,  relief,  cleavages,  inclusions,  alteration-products, 
etc.,  all  the  ordinary  rock-forming  minerals  can  be  identified 
in  most  cases  without  the  use  of  polarized  light.  If  the  lower 
nicol  is  not  readily  movable  it  may  be  left  in  for  many  purposes, 
but  it  must  be  remembered  that  half  the  illumination  is  thus 
cut  off,  and  for  any  but  the  lowest  magnifying  powers  this  is 
of  importance.  Opaque  substances  should  always  be  viewed 
in  reflected  light. 

To  examine  the  pleochroism  of  any  coloured  constituent, 
we  put  in  the  lower  nicol,  and  rotate  either  it  or  the  stage. 
For  verifying  feeble  pleochroism  the  former  plan  is  preferable, 
but  the  nicol  must  be  rotated  until  its  catch  holds  it  before 
proceeding  to  the  use  of  the  two  nicols,  which  will  be  the  next 
act. 

For  some  purposes  oblique  illumination  is  advantageous. 
For  instance,  the  extremely  slender  needles  of  apatite  in 

2—2 


20  CLASSIFICATION    OF   ROCKS. 

certain  lamprophyres  and  other  rocks  become  visible  only  by 
this  means.  A  'spot-lens'  may  be  improvised  by  placing 
beneath  the  stage  a  convex  lens  of  short  focal  length  with  its 
central  part  covered  by  a  disc  of  black  paper. 

In  using  a  high  power  it  will  be  noticed  that  the  focus  is 
very  perceptibly  different  for  the  upper  and  lower  surfaces  of 
the  slice.  To  make  out  the  form  of  a  body  enclosed  in  the 
thickness  of  the  slice  the  focus  should  be  gradually  moved,  so 
as  to  bring  different  depths  successively  into  view. 

It  cannot  be  too  strongly  insisted  that  the  identification 
of  the  component  minerals  of  a  rock  is  only  a  part  of  the 
examination.  The  mutual  relations  of  the  minerals  and  their 
structural  peculiarities  must  also  be  observed,  the  order  of 
crystallization,  intergrowths,  interpositions,  decomposition- 
products,  pseudomorphs,  etc. ,  as  well  as  special  rock-structures 
such  as  fluxion-phenomena,  vesicles,  effects  of  strain  and 
fracture,  etc.  In  short,  the  object  of  investigation  should 
be  not  merely  the  composition  of  the  rock,  but  its  history. 

Classification  and  nomenclature  of  rocks.  Petro- 
logy has  not  yet  arrived  at  any  philosophical  classification  of 
rocks.  Further,  it  is  easy  to  see  that  no  classification  can  be 
framed  which  shall  possess  the  definiteness  and  precision  found 
in  some  other  branches  of  science.  The  mathematically  exact 
laws  of  chemistry  and  physics  which  give  individuality  to 
mineral  species  do  not  help  us  in  dealing  with  complex  mineral 
aggregates;  and  any  such  fundamental  principle  as  that  of 
descent,  which  underlies  classification  in  the  organic  world, 
has  yet  to  be  found  in  petrology.  Rocks  of  different  types 
are  often  connected  by  insensible  gradations,  so  that  any 
artificial  classification  with  sharp  divisional  lines  cannot  truly 
represent  the  facts  of  nature.  At  present,  therefore,  the  best 
arrangement  is  that  which  brings  together  as  far  as  possible, 
for  convenience  of  description,  rocks  which  have  characters  in 
common,  the  characters  to  be  first  kept  in  view  being  those 
which  depend  most  directly  upon  important  genetic  conditions. 
The  grouping  adopted  below  must  be  regarded  as  one  of  con- 
venience rather  than  of  principle. 

In  a  perfect  system  the  nomenclature  should  correspond 
with  the  classification.  This  is  of  course  impossible  at  present 


NOMENCLATURE   OF   ROCKS.  21 

in  petrology.  Moreover  great  confusion  has  arisen  in  the 
nomenclature  of  rocks  in  consequence  of  the  rapid  growth  of 
descriptive  petrography.  Many  of  the  names  still  in  use  are 
older  than  the  modern  methods  of  investigation  :  they  were 
given  at  a  time  when  trivial  distinctions  were  emphasized, 
while  rocks  essentially  different  were  often  classed  together. 
Later  writers,  each  in  his  own  way,  have  arbitrarily  extended, 
restricted,  or  changed  the  application  of  these  older  names, 
besides  introducing  new  ones.  The  newer  rock-names  need 
cause  no  confusion,  provided  they  are  employed  in  a  strict 
sense.  Thus  '  foyaite '  should  be  used  for  rocks  like  that  of 
Foya,  specimens  of  which  are  in  every  geological  museum :  to 
extend  the  name  to  all  nepheline-bearing  syenites  is  to  intro- 
duce needless  ambiguity.  In  practice  perhaps  the  most  con- 
venient usage  is  to  speak  of  'the  Foya  type,'  'the  Ditro  type,' 
etc.,  referring  in  each  case  to  a  described  and  well-known  rock. 
There  remain  the  names  employed  for  families  of  rocks  :  some 
of  these  are  old  names,  such  as  granite  and  syenite,  which 
have  come  to  have  a  tolerably  well  understood  signification, 
not  always  that  first  attached  to  them  ;  others,  such  as  peri- 
dotite,  have  been  introduced  to  cover  rocks  not  recognized  as 
distinct  families  by  the  earlier  geologists.  A  division  of  a 
family  is  often  designated  by  prefixing  the  name  of  some 
characteristic  mineral  of  that  division ;  e.g.  hornblende-granite, 
hypersthene-andesite,  etc. 

These  remarks  apply  more  especially  to  igneous  rocks, 
which  we  shall  consider  first.  Such  rocks,  formed  by  the 
consolidation  of  molten  '  magmas,'  differ  from  one  another  in 
character,  the  differences  depending  partly  on  the  composition 
of  the  magma  in  each  case,  partly  on  the  conditions  attending 
its  consolidation.  The  composition  is  to  some  extent  indicated 
by  the  essential  minerals  of  the  rock,  which  thus  become  an 
important,  though  not  logically  a  prime,  factor  in  any  genetic 
classification.  It  is  evident,  however,  that  a  mere  enumeration 
of  the  minerals  of  a  rock,  without  taking  account  of  their 
relative  abundance,  cannot  give  a  very  precise  idea  of  the 
bulk -analysis 1 ;  while,  on  the  other  hand,  it  appears  on  examina- 
tion that  magmas  of  very  similar  composition  may,  under 

1  This  difficulty  is  only  partially  evaded  by  ranking  some  of  the  con- 
stituent minerals  as  essential  and  others  as  accessory. 


22  MAIN    DIVISIONS   OF    IGNEOUS   ROCKS. 

different  conditions  of  consolidation,  give  rise  to  widely  different 
mineral-aggregates.  Again,  many  rocks  consist  only  in  part 
of  definite  minerals,  the  residue  being  of  unindividualised 
matter  or  'glass.' 

To  diverse  conditions  of  consolidation  must  be  referred 
differences  in  coarseness  or  fineness  of  texture,  the  presence  or 
absence  of  any  glassy  residue,  the  evidence  of  one  or  more 
than  one  distinct  stage  in  the  solidification,  and,  in  general, 
the  peculiarities  in  the  mutual  arrangement  of  the  constituent 
minerals,  which  collectively  are  tenned  the  '  structure '  of  the 
rock. 

The  massive  igneous  rocks  will  first  be  divided  into  three 
•groups  :  abyssal  or  plutonic,  hypabyssal,  and  superficial  or 
volcanic.  These  names  express  the  different  geological  rela- 
tions of  the  several  groups  as  typically  developed,  but  the 
divisions  themselves  are  based  upon  the  characteristic  struc- 
tural features  which  different  conditions  of  consolidation  have 
impressed  upon  the  rocks.  Under  each  of  these  three  heads 
the  various  rock-types  will  be  grouped  in  families  founded 
proximately  on  the  mineralogical,  ultimately  on  the  chemical, 
composition,  though  this  cannot  be  done  without  some  few 
inconsistencies.  The  families  will  be  arranged  roughly  in 
order  from  the  more  acid  to  the  more  basic,  but  it  must  be 
remembered  that  such  an  arrangement  in  linear  series  can 
represent  only  very  imperfectly  the  manifold  diversity  met 
with  among  igneous  rocks. 


A.     PLUTONIC   ROCKS. 

THE  rock-types  to  be  treated  under  the  head  of  plutonic  or 
abyssal  are  met  with,  in  general,  in  large  rock-masses  which 
have  evidently  consolidated  at  considerable  depths  within  the 
earth's  crust.  Transgressive  as  regards  their  actual  upper 
boundary,  their  geological  relations  on  a  large  scale  are,  as  a 
rule,  only  imperfectly  revealed  by  erosion ;  so  that  their  actual 
form  and  extent  are  often  matters  of  conjecture.  Some  of  the 
masses  seem  to  be  of  the  nature  of  great  laccolites ;  others  have 
been  supposed  to  mark  reservoirs  of  molten  magma,  which  once 
furnished  the  material  of  minor  intrusions  and  surface  volcanic 
ejectamenta.  The  immediate  apophyses  of  the  large  masses 
have  similar  petrographical  characters. 

The  distinctive  features  of  these  rocks  of  deep-seated  con- 
solidation are  those  which  point  to  slow  cooling  (not  necessarily 
slow  consolidation)  and  great  pressure.  The  rocks  are  without 
exception  holocrystaUine,  i.e.  they  consist  wholly  of  crystallized 
minerals  with  no  glass.  Even  as  microscopic  inclusions  in  the 
crystals,  glass  is  much  less  characteristic  than  water,  which 
gives  evidence  of  high  pressure  during  the  crystallization.  The 
texture  of  plutonic  rocks  may  be  comparatively  coarse,  i.e.  the 
individual  crystals  of  the  essential  minerals  may  attain  con- 
siderable dimensions.  The  typical  structure  is  that  known  as 
hypidiomorphic,  only  a  minor  proportion  of  the  crystals  being 
'  idiomorphic '  (i.e.  developing  their  external  forms  freely), 
while  the  majority,  owing  to  mutual  interference,  are  more 
or  less  '  allotriomorphic '  (taking  their  shape  from  their  sur- 
roundings)1. It  should  be  observed  that  a  crystal  may  be 
strictly  idiomorphic  without  having  any  regular  crystal- 
outlines  :  this  is  often  the  case  with  olivine2. 

1  This  is  the  terminology  used  by  Rosenbusch.     Zirkel  has  adopted 
Eohrbach's  terms  automorphic  and  xenomorphic  in  the  same  senses. 

2  Pirsson  has  suggested  the  term  anhedron  (with  adjective  anhedral] 
for  a  crystal  not  possessing  external  crystal-faces :  Bull.  Geol,  Soc.  Amer. 
(1895)  vii,  492. 


24  SEQUENCE   OF   CRYSTALLIZATION. 

Sequence  of  crystallization.  The  terms  just  intro- 
duced are  used  with  a  relative  signification ;  so  that  a  given 
mineral  in  a  rock  may  be  allotriomorphic  towards  certain 
associated  minerals  and  idiomorphic  towards  others.  By 
observing  such  points  we  are  able  to  make  out  the  order  in 
which  the  several  minerals  composing  an  igneous  rock  have 
crystallized  out  from  the  parent  rock-magma.  It  is  found  that 
there  exists  in  plutonic  rocks  a  normal  order  of  consolidation 
for  the  several  constituents,  which  holds  good  with  a  high 
degree  of  generality.  It  is  in  the  main,  as  pointed  out  by 
Rosenbusch,  a  law  of  ( decreasing  basicity.'  The  order  is 
briefly  as  follows. 

I.  Minor  accessories  (apatite,  zircon,  sphene,  garnet,  etc.) 

and  iron-ores. 

II.  Ferro-magnesian  minerals  : — olivine,  rhombic  pyroxenes, 

augite,  aegirine,  hornblende,  biotite,  muscovite. 

III.  Felspathic  minerals : — plagioclase  felspars  (in  order  from 

anorthite  to  albite),  orthoclase  (and  anorthoclase). 

IV.  Quartz,  and  finally  microcline. 

In  most  rocks  such  minerals  as  are  present  follow  the 
above  order.  The  most  important  exceptions  are  the  inter- 
growth  of  orthoclase  and  quartz  and  the  crystallization  of 
quartz  in  advance  of  orthoclase  in  some  acid  rocks,  and  the 
rather  variable  relations  between  groups  II.  and  III.  in  some 
more  basic  rocks.  The  order  laid  down  applies  in  general  to 
parallel  intergrowths  of  allied  minerals  :  thus  when  augite  is 
intergrown  with  regirine  or  hornblende,  the  former  mineral 
forms  the  kernel  of  the  complex  crystal  and  the  latter  the 
outer  shell ;  when  a  plagioclase  crystal  consists  of  successive 
layers  of  different  compositions,  the  layers  become  progressively 
more  acid  from  the  centre  to  the  margin. 

Certain  constituents  having  variable  relations  are  omitted 
from  the  foregoing  list.  Thus  nepheline  (elreolite)  and  sodalite 
belong  to  group  III.,  but  may  crystallize  out  either  before  or 
after  the  felspars. 

Varieties   of  structure   in   plutonic   rocks.     The 

typical  structure  of  rocks  of  plutonic  habit  is  that  implied 
in  the  foregoing  remarks,  and  is  known  as  the  granitoid  or 


STRUCTURES   OF   PLUTONIC   ROCKS.  25 

eugranitic  structure1.  Among  the  more  special  modifications 
frequently  met  with  are  those  depending  upon  the  simul- 
taneous crystallization  of  two  of  the  essential  minerals,  giving 
rise  to  the  so-called  'graphic'  intergrowths,  usually  on  a  micro- 
scopic scale.  The  resulting  micrographic,  micropegmatitic  or 
granophyric  structure  is  most  common  in  the  quartz-bearing 
rocks,  and  arises  there  from  an  intimate  interpenetration 
of  part  of  the  felspar  by  quartz  (fig.  7,  A\  Within  a  certain 
area  of  a  slice  the  quartz  of  such  an  intergrowth  behaves 
optically  as  if  it  were  a  single  crystal,  the  whole  becoming 
dark  between  crossed  nicols  in  one  position.  On  rotation  the 
felspar  can  be  made  to  extinguish  in  its  turn.  Intergrowths 
of  other  minerals  (e.g.  augite  and  felspar)  are  less  common. 
In  both  granitoid  and  micrographic  rocks  there  sometimes 
occur  vacant  interstitial  spaces  or  little  cavities  of  irregular 
shape,  into  which  project  the  sharp  angles  of  well-formed 
crystals.  Such  rocks  are  said  to  have  a  miarolitic  or  drusy 
structure,  but  this  peculiarity  is  often  obscured  by  secondary 
products  occupying  the  druses. 

Opposed  to  the  granitoid  is  the  granulitic  structure.  In 
this  a  section  of  the  rock  appears  as  a  mosaic  of  roughly 
equidimensional  grains,  usually  of  small  size.  There  is  only 
in  some  cases  a  tendency  to  crystallographic  development 
(panidiomorphic  structure)  or  again  the  earlier-formed  minerals 
tend  to  take  on  rounded  outlines.  The  structure  probably 
results  from  movement  during  the  process  of  consolidation, 
and  we  shall  see  that  very  similar  appearances  may  be 
produced  by  the  deformation  and  crushing  of  already  solidified 
granitoid  rock-masses. 

Both  granitoid  and  granulitic  rocks  sometimes  exhibit  in 
greater  or  less  degree  a  parallel  disposition  of  elongated  or 
tabular  crystals  of  felspar,  mica,  etc.,  indicative'  of  some 
flowing  movement  of  the  rock-magma  subsequently  to  the 
separation  of  those  crystals.  With  this  there  may  be  a 
certain  banding  of  the  rock  due  to  alternations  of  slightly 
different  types  (mineralogically  or  structurally),  which  is 
known  as  a  gneissic  structure.  These  characters,  however, 

1  The  term  'granolite,'  applied  to  such  rocks  by  some  American 
writers,  is  ill  chosen,  as  likely  to  be  confused  with  'granulite.' 


26         SPECIAL   MODIFICATIONS   OF   PLUTONIC   ROCKS. 

may  also  have  a  quite  different  and  secondary  origin,  and  we 
shall  defer  notice  of  them  to  another  place  (Chap.  XXII.). 

Traversing  plutonic  rock-masses  of  normal  structural  types, 
or  bordering  them  as  an  irregular  fringe,  may  often  be  found 
strikingly  coarse-textured  or  pegmatitw  modifications,  with 
a  strong  tendency  to  graphic  intergrowths1.  While  clearly 
related  to  the  associated  plutonic  rock-masses,  these  pegmatitic 
rocks  differ  from  them  mineralogically  in  the  sense  of  being 
somewhat  more  acid,  and  they  are  further  characterized  by 
the  frequent  occurrence  of  special  minerals,  often  including 
compounds  of  the  rarer  chemical  elements.  They  are  usually 
regarded  as  representing  the  final  (pneumatolytic)  phase  of 
consolidation  of  the  rock-magmas  from  which  they  were 
formed2.  The  lighter-coloured  veins  and  streaks  often  seen 
traversing  plutonic  rocks  are  in  many  respects  comparable 
with  the  pegmatites.  They  invariably  shew  a  coarser  texture 
and  a  more  acid  composition  than  the  main  mass  in  which 
they  occur;  and,  though  they  more  or  less  clearly  cut  the 
latter,  the  relations  are  such  as  to  prove  that  their  origin 
is  bound  up  with  that  of  the  main  rock-mass.  They  are 
sometimes  spoken  of  as  (relatively)  acid  excretions  from  the 
crystallizing  magma. 

Contrasted  with  these,  there  occur  in  many  plutonic  rocks 
darker  and  finer-textured  ovoid  or  irregularly  rounded  patches 
which  are  usually  considered  as  (relatively)  basic  secretions 
from  the  magma,  belonging  to  an  early  stage  in  the  history  of 
consolidation.  Composed  in  general  of  the  same  minerals  as 
the  enclosing  rock,  they  are  richer  in  the  earlier-formed — 
which  are  also  the  denser  and  more  basic— constituents.  The 
lighter-coloured  veins,  on  the  other  hand,  are  relatively  rich  in 
the  later-formed  and  more  acid  minerals. 

The  typical  plutonic  rocks  are  non-porphyritic,  i.e.  there 
is  evidence  of  but  one  continuous  stage  in  the  consolidation. 
In  many  hypabyssal  and  almost  all  volcanic  rocks,  some  one, 

1  The  original  pegmatite  of  Haiiy  was  such  an  intergrowth  of  quartz 
and  felspar  ('graphic  granite'),  but  the  modern  usage  of  the  name  is  more 
extended. 

2  On  this  point  see  G.  H.  Williams,  15th  Ann.  Rep.  U.  S.  Geol.  Snr. 
(1895)  675-684. 


PORPHYRITIC   STRUCTURE.  27 

or  more,  constituent  (usually  a  felspar)  occurs  in  two  distinct 
generations  with  different  habits  and  characters,  belonging  to 
an  earlier  and  a  later  stage  of  consolidation,  in  which  quite 
different  conditions  prevailed.  This  is  the  '  porphyritic ' 
structure,  and  is  typically  wanting  among  plutonic  rocks, 
which  have  what  has  been  termed  an  *  even-grained '  character 
('kornig'  of  Rosenbusch).  In  some  of  the  plutonic  rocks, 
however,  and  especially  among  the  granites,  occur  relatively 
large  crystals  of  felspar,  which  give  a  porphyritic  character 
to  the  rock  of  which  they  form  part,  and  perhaps  point  to 
different  conditions  from  those  under  which  the  main  mass  of 
the  rock  consolidated ;  but  even  here  there  is  no  sharp  division 
between  an  earlier  and  a  later  period  of  crystallization,  such 
as  is  indicated  in  the  volcanic  rocks l. 

We  shall  consider  the  several  families  in  an  order  which 
corresponds  roughly  with  their  chemical  relationship,  beginning 
with  the  acid  rocks  and  ending  with  the  ultrabasic. 

1  Cf.  Lawson  on  the  Santa  Lucia  granite  in  California,  Bull.  Dep. 
Geol  Univ.  Gal.  (1893)  i,  9—15. 


CHAPTER  II. 

GRANITES. 

THE  granites  are  even-grained  holocrystalline  rocks  com- 
posed of  one  or  more  alkali-felspars,  quartz,  and  some  ferro- 
magnesian  mineral,  besides  accessory  constituents.  The  rocks 
are  generally  of  medium  to  rather  coarse  grain,  and  the 
tendency  of  the  crystals  as  a  whole  to  interfere  with  one 
another's  free  development  gives  what  Rosenbusch  styles  the 
hypidiomorphic  structure. 

According  to  their  characteristic  minerals,  after  felspars 
and  quartz,  the  rocks  are  described  as  muscovite-,  biotite-, 
hornblende-,  and  augite-granites ;  and  this  division  corresponds 
roughly  with  different  chemical  compositions,  from  more  to  less 
acid  types.  Tourmaline-granite  must  be  considered  a  special 
modification  of  the  above,  and,  in  particular,  of  the  more  acid 
kinds.  With  the  granites  we  shall  also  include  certain  rocks 
(aplite,  pegmatite,  greisen)  associated  with  granites  but  differ- 
ing from  them  in  important  structural  and  mineralogical 
characters,  some  of  them  never  forming,  like  the  true  granites, 
large  bodies  of  rock. 

Constituent  minerals.  Felspars  make  up  the  greater 
part  of  a  granite,  a  potash-  and  a  soda-bearing  felspar  commonly 
occurring  together.  The  potash-felspar  is  often  orthoclase, 
either  in  simple  crystals  or  in  Carlsbad  twins,  the  Baveno 
twin  being  uncommon1.  When  fresh,  it  shews  its  cleavages 
and  sometimes  a  slight  zonary  banding,  but  these  appearances 
are  lost  when  the  mineral  is  altered  to  any  extent.  The 

1  Cohen  (3),  pi.  xxiv,  fig.  2 ;  Kosenbusch-Iddings,  pi.  xxm,  fig.  3. 


FELSPARS   OF   GRANITES. 


29 


common  decomposition-processes  give  rise  either  to  finely 
divided  kaolin  or  to  minute  Hakes  of  mica.  When  the  latter 
are  large  enough  to  be  clearly  distinguished,  they  are  often 
seen  to  lie  along  the  cleavage-planes  of  the  felspar.  Decom- 
position often  begins  in  the  interior  of  a  crystal,  which  may 
be  clouded  or  completely  obscured  while  the  margin  remains 
clear.  Instead  of  orthoclase  we  often  find  microcline^  which 
is  usually  the  last  product  of  consolidation  in  the  rock.  When 
fresh,  microcline  shews  its  characteristic  'cross-hatched'  struc- 
ture1 and  sometimes  a  vein-like  intergrowth  of  albite2  (fig.  4). 


a, 


FIG.  4.     MICROCLINE  FROM  THE  'BAPAKIWI'  GRANITE  OF  FINLAND;   x  20. 
Crossed  nicols :   shewing  the  characteristic  'cross-hatching.'    It  is 
traversed  by  veinlets    of    albite   (a)    intergrown   with    crystallographic 
relation  to  the  microcline  [1031]. 

Some  petrologists  hold  that  the  peculiar  microcline- structure, 
due  to  fine  twin-lamellation  in  two  directions,  is  not  essential, 
and  may  be  set  up  in  some  cases  as  a  secondary  effect  of 
strain ;  and  that  the  quasi-inonoclinic  mineral  orthoclase  is 
merely  microcline  in  which  the  twin-lamellation  is  carried  to 
an  ultra-microscopic  degree  of  fineness3.  The  alteration  of 

1  Cohen  (3),  pi.  xxvm,  figs.  1,  2 ;  pi.  xxxn,  fig.  4. 

2  Rosenbusch-Iddings,  pi.  xxv,  fig.  1. 

3  Cf.  Teall,  Ann.  Rep.  Geol.  Sur.  for  1895,  p.  24 ;  Keyes,  15th  Ann. 
Rep.  U.  S.  Geol.  Sur.  (1895)  711,  712. 


30  QUARTZ    OF   GRANITES. 

microcline  by  weathering  is  similar  to  that  of  orthoclase. 
The  soda-felspar  of  most  granites  ranges  from  albite  to 
oligoclase.  It  has  rather  a  tabular  habit,  giving  rise  to 
elongated  rectangular  sections.  It  is  always  twinned  on  the 
albite-  and  occasionally  too  on  the  pericline-law.  The  com- 
mon decomposition-products  are  kaolin,  sometimes  paragonite 
mica,  and  in  the  lime-bearing  varieties  some  epidote  or  calcite. 
The  most  typical  'soda-granites'  contain  albite  to  the  exclusion 
of  potash-felspars,  but  this  is  an  exceptional  type  (Croghan 
Einshela  in  Wexford,  Mariposa  in  the  Sierra  Nevada).  Parallel 
intergrowths  of  orthoclase  and  plagioclase  are  sometimes  found 
(microperthite).  The  felspars  of  granite  are  not  rich  in  in- 
clusions, but  they  may  inclose  sparingly  microlites  of  the 
earlier  constituents  of  the  rock. 

The  quartz  of  granites  does  not  usually  shew  any  crystal 
boundaries,  except  on  the  walls  of  drusy  cavities  ('  miarolitic ' 
structure),  or  less  perfectly  when  the  mineral  is  enclosed  by 
microcline.  Its  most  characteristic  inclusions  are  fluid-cavities 
(fig.  1,  b — e):  these  are  sometimes  in  the  form  of  'negative 
crystals,'  either  dihexahedral  pyramids  or  elongated  prisms ; 
more  usually  the  shape  is  rounded  or  irregular.  These  fluid- 
pores  often  occur  with  a  definite  arrangement  along  certain 
planes,  appearing  in  a  section  as  lines1.  The  enclosed  liquid 
does  not  fill  the  cavity,  but  leaves  a  bubble,  which  is  mobile. 
In  some  cases  the  liquid  is  brine,  and  contains  minute  cubes  of 
rock-salt  (Dartmoor).  In  others  liquid  carbonic  acid  occurs 
instead  of,  or  in  addition  to,  water,  and  in  some  cases  we  see 
one  bubble  within  another2.  Glass-  and  stone-cavities  are  less 
abundant.  Sometimes  extremely  fine  needles  are  enclosed 
(Peterhead)  :  these  seem  to  be  rutile,  and  sometimes  shew  the 
characteristic  knee-shaped  twin. 

The  dark  micas  of  granites  are  usually  termed  biotite. 
This  may  be  considered  to  include  varieties  rich  in  ferrous 
oxide  (the  haughtonite  of  many  Scottish  and  Irish  granites), 
or  in  ferric  oxide  (lepidornelane).  The  mineral  builds  roughly 
hexagonal  plates,  which,  cut  across,  give  an  elongated  section 
shewing  the  strong  basal  cleavage3.  A  lamellar  twinning 

1  Cohen  (3),  pi.  xi,  fig.  1.  2  Ibid.  pi.  xn,  fig.  3. 

3  Ibid.  pi.  XL,  fig.  2. 


MICAS   OF   GRANITES. 


31 


parallel  to  the  base  is  probably  common,  but,  owing  to  the 
nearly  straight  extinction,  this  is  not  often  conspicuous.  The 
fresh  biotite  is  deep  brown  with  intense  pleochroism.  Its 
common  inclusions  are  apatite,  zircon,  and  magnetite,  and  the 
minute  zircons  are  always  surrounded  by  a  '  halo '  of  extremely 
deep  colour  and  intense  pleochroism1  (Skiddaw,  Dartmoor, 
Dublin,  etc.).  Decomposition  often  produces  a  green  colora- 
tion2 and  ultimately  a  green  chloritic  pseudomorph  with 
secondary  magnetite-dust.  This  magnetite  may  be  reabsorbed, 
restoring  the  brown  colour  but  with  less  pleochroism  and  with 
loss  of  cleavage. ' 


^>>^      B 


FIG.  5.     GRANITE,  NEAK  DUBLIN  ;    x  20. 

A.  Crystal  of  oligoclase  shewing  zonary  structure  and  decomposition 
beginning  in  the  interior  [389].  B.  Parallel  intergrowth  of  biotite  and 
muscovite  [1774]. 


The  colourless,  brilliantly-polarizing  muscovite  forms  rather 
ragged  flakes,  posterior  to  the  biotite  or  partly  in  parallel 
intergrowth3  with  it  (Dublin,  etc.,  fig.  5,  B).  It  is  always  clear, 
and  is  not  susceptible  to  weathering.  A  lithia-mica,  in  large 

1  Cohen  (3),  pi.  LIV,  figs.  3,  4.  2  Ibid.  pi.  LVII. 

3  Ibid.  pi.  xxxii,  fig.  2. 


32  DARK   MINERALS   OF   GRANITES. 

flakes,  takes  the  place  of  muscovite  in  some  greisens  and  peg- 
matites. 

The  crystals  of  hornblende  are  irregularly  bounded,  or  at 
least  without  terminal  planes.  *  They  shew  the  prismatic 
cleavage,  and  occasionally  lamellar  twinning  parallel  to  the 
orthopinacoid.  The  colour  is  green  or  brownish-green,  with 
marked  pleochroism,  and  the  extinction-angle  in  longitudinal 
sections  always  low.  Besides  inclusions  of  earlier  minerals, 
there  may  be  an  intergrowth  with  biotite.  The  common 
decomposition-products  are  a  green  chloritic  substance  or  an 
epidote  and  quartz. 

When  augite  occurs,  it  is  commonly  the  variety  malacolite 
or  diopside,  colourless  in  slices.  It  is  not  usually  in  perfect 
crystals,  but  an  idioinorphic  green  augite  is  found  in  some 
coarsely  granophyric  types  of  rock  (Mull).  Augite  may  be 
either  uralitized  or  decomposed  into  a  green  chloritic  mineral 
or  into  a  mixture  of  serpentine  and  calcite.  The  augite  is 
sometimes  accompanied  by  a  rhombic  pyroxene  (enstatite, 
Cheviot),  and  in  one  remarkable  group  of  granitic  rocks  the 
dominant  ferro-magnesian  element  is  hypersthene. 

Iron-ores  are  not  plentiful  in  granites.  Magnetite  may 
occur  or  kwmatite,  either  opaque  or  deep-red ;  pyrites  is  also 
found  as  an  original  mineral. 

Acute-angled  crystals  of  light-brown  pleochroic  sphene 
(titanite)  are  often  seen,  and  in  the  less  acid  granites  are 
abundant  (fig.  7,  B).  Eounded  grains  may  occur  instead. 
The  high  refractive  index  and  other  optical  properties  enable 
the  .mineral  to  be  readily  identified.  The  little  prisms  of 
zircon^  are  even  more  highly  refractive  (tig.  1,  £),  but  when 
they  occur,  as  they  often  do,  enclosed  in  the  biotite,  the 
pleochroic  halo  is  liable  to  obscure  their  nature.  Apatite 
builds  narrow  colourless  prisms,  and  often  penetrates  the 
biotite.  Small  reddish  garnets  occur  in  some  muscovite- 
granites  and  aplites  (Dublin)  :  other  unusual  minerals  are 
cordierite,  usually  pseudomorphed  by  the  micaceous  substance 
termed  pinite,  and  andalusite,  coated  with  flakes  of  muscovite. 
In  some  granites  from  America  and  elsewhere  allanite  (orthite) 

1  Cohen  (3),  pi.  in,  fig.  3. 


TOURMALINE   OF   GRANITES. 


33 


is  found1,  while  others  contain  epidote,  often  with  an  inter- 
grown  core  of  allanite2.  Though  epidote  is  a  well-known 
weathering-product  in  granitic  rocks,  this  relation  to  allanite 
and  the  occasional  inclusion  of  good  crystals  of  epidote  in 
flakes  of  biotite  seem  to  point  to  its  primary  origin  in  these 


cases' 


Tourmaline  characterizes  a  common  modification  of  granite, 
especially  near  the  margin  of  a  mass.  It  may  be  in  good 
crystals  but  more  frequently  has  ragged  outlines.  The  rude 


B 


FIG.  6.    REVERSALS  OF  NORMAL  ORDER  OF  CRYSTALLIZATION  IN 

GRANITES  ;    X  20. 

A.  Biotite  moulded  on  muscovite,  Rubislaw,  Aberdeen  [390  a], 
B.  Biotite  moulded  on  quartz  and  felspars,  Meillionydd,  near  Sarn, 
Caernarvonshire  [814].  C.  Orthoclase  moulded  on  quartz,  Shap  [892]. 

cross-fracture  is  often  apparent.     The  colour  is  brown,  some- 
times with  patches  of  blue4,  and  the  dichroism  is  strong,  the 

1  Iddings  and  Cross,  A.  J.  S.  (1885)  xxx,  108-111;  McMahon,  O.  M. 
1899,  194-196  (Lairg). 

2  Hobbs,  A.  J.  S.  (1889)  xxxviii,  223-228 ;   Amer.  Geol.  (1893)  xii, 
218,  219. 

3  Hobbs,  Amer.  Geol.  (1893)  xii,  218,  219;   Keyes,  Bull.  Geol.  Soc. 
Amer.  (1893)  vi,  305-312;  15th  Ann.  Rep.  U.  S.  Geol.  Sur.  (1895)  704- 
710,  pi.  xxxviii,  figs.  1-4,  xxxix,  figs.  1-3,  XL. 

4  Cohen  (3),  pi.  xxn,  fig.  2. 

H.   P.  3 


34  STRUCTURE   OF   GRANITES. 

strongest  absorption  being  for  vibrations   transverse   to  the 
long  axis  (the  'ordinary'  ray). 

Structure.  In  the  granites  the  normal  order  of  crystal- 
lization of  the  constituent  minerals  rules  with  few  exceptions. 
The  minor  accessory  minerals  crystallized  out  first,  and  are 
thoroughly  idiomorphic,  i.e.  have  taken  their  shape  without 
external  interference.  The  ferro-magnesian  minerals  have  in 
general  preceded  the  felspars,  being  often  embraced  or  even 
enclosed  by  them,  though  the  felspars  also  may  tend  to  take 
on  partial  crystal-outlines.  Rarely  does,  e.g.,  mica  occur 
interstitially  to  felspar  (fig.  6,  B).  Biotite  moulded  on  mus- 
covite  is  not  so  rare  (fig.  6,  A).  Apart  from  micrographic 
structures,  the  felspars,  except  microcline,  have  crystallized 
prior  to  the  quartz,  exceptions  being  infrequent  (fig.  6,  (7). 
Where  micrographic  intergrowths  occur,  the  felspar  may  be 
either  orthoclase  or  a  plagioclase  (fig.  7,  A).  We  need  not 
further  specify  other  structural  peculiarities  such  as  the  miaro- 
litic,  the  porphyritic,  the  gneissic,  and  the  spheroidal  or 
orbicular1. 

Leading  types.  Almost  all  the  true  granites  contain 
a  brown  mica.  If  a  white  mica  be  present  in  addition,  we 
have  muscovite- granite  ('two-mica  granite'  or  'granite  proper' 
of  the  Germans,  'granulite'  of  the  French2,  'binary  granite' 
of  some  American  writers3).  Such  rocks  are  commonly 
somewhat  more  acid  in  composition  than  those  with  dark 
mica  only.  The  Carboniferous  granites  of  Cornwall  and 
Devon  afford  good  examples.  They  consist  of  orthoclase,  a 
plagioclase,  quartz,  and  two  micas4,  with  the  normal  order  of 
crystallization.  The  quartz  has  fluid-cavities,  often  enclosing 

1  Hatch,  Q.  J.  G.  S.  (1888)  xliv,  548-559,   pi.  xiv  (Mullaghderg  in 
Donegal),   with  a  summary  of  information  on   spheroidal   granites   in 
general ;  Turner,  Journ.  Geol.  (1899)  vii,  154  (Bridal  Veil  in  Yosemite 
Park) ;  Harris,  G.  M.  1898,  11-13  ('  Bapakiwi '  granite  of  Finland). 

2  The  granulite  of  German  and  English  petrologists  has  a  different 
signification. 

3  This  term,  however,  has  also  been  applied  to  rocks  consisting  essen- 
tially of  felspar  and  quartz,  without  mica. 

4  Dr  Haughton's  analyses  of  the  Trewavas  Head  rock  proved  the 
felspar  to  be  albite,  the  dark  mica  lepidomelane,  and  the  white  mica 
lepidolite ;  Q.  J.  G.  8.  (1869)  xxv,  166,  167. 


MUSCOVITE-GRANITES.  35 

minute  cubes  of  rock-salt1  (Dartmoor,  fig.  1,  c).  Parallel 
intergrowths  of  biotite  and  muscovite  are  common.  The 
minor  constituents  of  the  rock  are  magnetite,  apatite,  and 
zircon,  the  last,  when  it  is  enclosed  in  the  biotite,  being  always 
encircled  by  the  characteristic  halo  of  intense  pleochroism. 
More  exceptional  accessory  minerals  are  andalusite,  in  pleo- 
chroic  crystals  coated  by  flakes  of  muscovite  (Cheesewring), 
and  'pinite'  pseudomorphs  after  cordierite  (Land's  End). 
Tourmaline  is  common,  and  the  rocks  graduate  into  tourma- 
line-granites, especially  near  the  margin  of  an  intrusion. 

The  post-Ordovician  granites  which  occupy  so  large  a  tract 
in  Leinster8  (e.g.  Dalkey  near  Dublin)  are  of  a  different  type. 
They  also  have  two  micas,  often  in  parallel  intergrowth,  and 
apatite  and  zircon  are  characteristic  accessories ;  but  the 
potash-felspar  is  microcline3,  and  is  the  latest  product  of  crystal- 
lization. A  plagioclase  felspar  is  plentiful,  and  exceptionally 
albite  is  the  only  felspathic  element  present  (Croghan  Kinshela 
in  Wexford).  Little  crystals  of  garnet  occur  in  some  instances 
(Three  HOCK  Mountain  near  Dublin).  This  mineral  is  found 
also  in  the  granite  of  Foxdale  in  the  Isle  of  Man4,  a  closely 
similar  rock,  in  which  the  dark  mica  is  very  subordinate  to 
the  white.  Another  well-known  microcline-bearing  rock  is 
the  '  grey  Aberdeen  granite '  of  Rubislaw,  etc.  Similar  rocks 
are  found  in  Donegal. 

Among  American  muscovite-granites  may  be  mentioned 
those  of  Concord  and  Haberville,  N.H.,  and  the  porphyritic 
granite  of  Coanicut  Island,  K.I.5  Others  occur  in  Maine, 
Vermont,  and  Connecticut. 

Rocks  in  which  muscovite  is  only  sparingly  or  occasionally 
present  form  a  link  with  the  next  division.  The  Skiddaw 
granite  is  of  this  character6.  Here  the  quartz  is  in  great 
part  of  prior  crystallization  to  the  orthoclase,  or  there  may  be 

1  Hunt,  G.  M.  1894,  102-104,  with  figures. 

2  Sollas,  Trans.  Roy.  Ir.  Acad.  (1891)  xxix,  427-512 ;  Pr.  Ge.ol.  Ass. 
(1893)  xiii,  106  ;  Watts,  Guide,  31-33. 

3  O'Reilly,  Sci.  Pr.  Roy.  Dub.  Soc.  (1879)  ii,  246-248,  pi.  xv. 

4  Naturalist,  1894,  68;  Q.  J.  G.  S.  (1895)  li,  143. 

5  Pirsson,  A.  J.  S.  (1893)  xlvi,  372,  373. 

6  Q.  J.  G.  S.  (1895)  li,  140. 

3—2 


36 


BIOTITE-GRANITES. 


some  micrographic  intergrowth  of  the  two  minerals.  Felspar- 
quartz-rocks  free  from  mica  are  found  among  the  pre-Cambrian 
intrusions  of  Ercal  in  the  Wrekin  district  and  of  the  Malverns. 
Here  too  the  quartz  has  crystallized,  or  lias  finished  crystal- 
lizing, before  the  dominant  felspar,  which  is  often  microcline. 
These  rocks  seem  to  have  affinities  with  the  pegmatites. 

The  commonest  division  of  the  granite  family  is  perhaps 
biotite-granite  (Fr.  granite,  Ger.  Granitit),  characterized  by 
containing  a  brown  mica  to  the  exclusion  of  inuscovite,  horn- 
blende, or  augite.  Such  a  rock  may  consist,  e.g.,  of  orthoclase, 
albite  or  oligoclase,  quartz,  biotite,  and  minor  accessories,  with 
the  normal  order  of  crystallization. 

The  relative  proportions  of  the  several  minerals  vary 
considerably.  In  the  granites  (Ordovician  and  perhaps  some 


B 


older)  of  Wales1  quartz  is  very  abundant,  and  biotite  (often 
chloritized)  is  only  sparingly  found.     The  dominant  felspar 

i  Q.  J.  G.  S.  (1888)  xliv,  444,  445,  and  Bala  Vole.  Ser.  Caern.  59,  61 
(Sara) ;  Geikie,  Q.  J.  G.  S.  (1883)  xxxix,  314,  pi.  x,  fig.  11  (St  David's) ; 
Jennings  and  Williams,  ibid.  (1891)  xlvii,  380  (Ffestiniog). 


BIOTITE-  GRANITES.  37 

is  often  a  plagioclase  (Caernarvon,  St  David's,  etc.),  and 
probably  some  of  these  rocks  would  be  placed  among  the  '  soda- 
granites  '  of  certain  authors.  The  St  David's  rock  shews  a 
strong  tendency  to  the  micrographic  structure  (fig.  7,  A). 

In  the  biotite-granite  of  Eskdale,  Cumberland,  the  quartz 
is  either  intergrown  in  micrographic  fashion  with  the  ortho- 
clase,  or  has  crystallized  before  it.  The  latter  is  the  case  too 
in  the  well-known  porphyritic  granite  of  Shap  in  Westmor- 
land1 (fig.  6,  C\  which  is  further  noteworthy  for  its  abundant 
sphene.  Both  micrographic  and  miarolitic  structures  charac- 
terize the  Tertiary  biotite-granites  of  the  Mourne  Mts, 
Carlingford2,  and  Arran,  the  crystals  on  the  walls  of  the 
druses  presenting  very  perfect  crystal  boundaries. 

Biotite-granites  are  extensively  developed  in  the  Cairngorm15 
and  Monadhliath  Mts  and  other  parts  of  the  Scottish  High- 
lands. In  many  British  examples  microcline  partly  or  wholly 
takes  the  place  of  orthoclase  (Malvern,  Ross  of  Mull,  Peter- 
head,  etc.).  Albite-veins  intergrown  in  both  orthoclase  and 
microcline  may  sometimes  be  observed,  e.g.  in  the  Eskdale  rock 
already  alluded  to. 

Biotite-granites  are  of  wide-spread  occurrence  in  the 
Atlantic  States  of  America4,  as  well  as  in  Nova  Scotia  and 
New  Brunswick.  In  Maine  five-sixths  of  the  granitic  rocks 
belong  to  this  division.  Several  varieties  are  described  by 
Kemp  from  Rhode  Island  and  Connecticut,  some  containing 
allanite  (Westerly),  garnet  (Stony  Creek),  and  other  special 
minerals.  In  some  of  these  rocks  microcline  is  a  prominent 
constituent,  as  also  in  biotite-granites  from  Central  Maryland5 
and  Alabama6.  The  granite  of  Ilchester,  Md.,  contains  primary 


1  Teall,  pi.  xxxv,  fig.  1  [395] ;  Barker  and  Marr,  Q.  J.  G.  S.  (1891)  xlvii, 
275-285,  pi.  xi,  fig.  1. 

2  Sollas,  Trans.  Roy.  Ir.  Acad.  (1894)  xxx,  490. 

3  Craig,  Summary  of  Progress,  Geol.  Sur.  1898,  28,  and  1900,  22. 

4  Kemp,  Bull.  Geol.  Soc.^Amer.  (1899)  x,  377-382. 

5  Keyes,  15th  Ann.  Rep.  U.  S.  Geol.  Sur.  (1895)  696-730. 

6  Clements,  Butt.  5  Geol.  Sur.   Ala.  (1896)  139-142;   Brooke,  ibid 
185,  186. 


38  HORNBLENDE-GRANITES. 

epidote  with  allanite1.  Coarse-grained  porphyritic  granites  are 
extensively  developed  in  Georgia2,  and  similar  rocks  occur  in 
North  and  South  Carolina.  They  consist  of  quartz,  orthoclase 
(with  microperthite),  microcline,  oligoclase,  and  a  variable 
amount  of  biotite. 

Less  abundant  than  the  types  characterized  by  micas,  and 
usually  of  less  acid  composition,  is  hornblende-granite  (Ger. 
Amphibolgranit),  in  which  the  distinctive  mineral  is  a  green 
hornblende,  usually  with  biotite  in  addition.  Some  of  the 
newer  Palaeozoic  granites  of  Scotland  are  of  this  kind,  such 
as  that  of  Lairg3  and  Ord  Hill4  in  Sutherland  and  the  Criffel 
rock  at  Dalbeattie5,  in  which,  however,  biotite  often  pre- 
dominates. The  Criffel  granite,  with  others  in  Galloway, 
graduates  into  a  quartz-diorite.  The  hornblende-granite  of 
Loch  Etive  is  coarse-grained,  and  has  porphyritic  crystals 
of  orthoclase.  The  rock  quarried  at  Mount  Sorrel  in  Charn- 
wood  Forest,  Leicestershire6,  is  also  in  part  a  hornblende- 
granite,  having  that  mineral  associated  with  biotite.  In 
Ireland  a  hornblende-granite  has  been  described  from  Donegal 7, 
and  another  is  associated  with  the  Palaeozoic  biotite-granites 
of  Newry  (at  Goragh  Wood).  Hornblende-granites  of  Tertiary 
age  are  found  in  Skye  and  Mull.  In  these  the  brownish  green 
hornblende  is  associated  with  subordinate  biotite.  The  rocks 
often  shew  a  rude  micrographic  structure,  and  graduate  into 
typical  granophyres,  in  which  the  biotite,  and  to  some  extent 
the  hornblende,  give  place  to  a  greenish  augite.  A  miarolitic 
structure  is  common,  the  cavities  often  obscured  by  calcite 
and  other  secondary  products. 

Hornblende-granites,  often  rich  in  sphene,  are  largely 
developed  in  Nevada  and  Utah8.  In  Massachusetts  the 

1  Hobbs,  A.  J.  S.  (1889)  xxxiii,  223-228. 

2  Watson,  Journ.  of  Geol.  (1901)  ix,  97-122. 

3  Heddle,  M.  M.  (1883)  v,  178-184;  Cole's  Stud.  Micro.  Sci.  No.  42 
(plate) ;  McMahon,  G.  M.  1899,  194-196. 

4  Cole's  Stud.  Micro.  Sci.  No.  38  (plate). 

5  Teall,  Mem.  Geol.  Sur.  Scot.,  Expl.   Sheet  5  (1896)  41-43  ;   Ann. 
Rep.  Geol.  Sur.  for  1896,  41-44 ;  and  Mem.  Geol.  Sur.,  Silur.  Rocks  Scot. 
(1899)  507-525. 

6  Bonney,  Q.  J.  G.  S.  (1878)  xxxiv,  219. 

7  Hatch,  ibid.  (1888)  xliv,  548-551. 

8  Zirkel,  Micro.  Petrog.  Fortieth  Parallel  (1876)  40-52. 


AUGITE-   AND   HYPERSTHENE-GRANITES.  39 

Rockport  granite  is  a  well-known  example  ;  that  of  Cape  Aim 
has  subordinate  augite  with  the  hornblende  and  biotite,  and 
allanite  as  an  accessory1 ;  that  of  Quincy  has  instead  of  horn- 
blende the  deep  blue  amphibple-mineral  riebeckite3,  which  has 
also  been  described  by  Lacroix  from  St  Peter's  Dome,  El  Paso, 
Colorado.  The  Albany  granite3,  in  New  Hampshire,  carries 
porphyritic  crystals  of  orthoclase  with  perthitic  intergrowths 
of  albite:  biotite,  hornblende,  and  sometimes  pyroxene  are 
present,  and  zircon  is  a  conspicuous  accessory. 

If  we  exclude  the  granophyric  varieties,  augite-granite  is 
by  no  means  an  abundant  rock-type.  An  example,  of  Old  Red 
Sandstone  age,  occurs  in  the  Cheviots4.  This  consists  of 
orthoclase,  plagioclase,  quartz,  augite,  exceptionally  enstatite, 
biotite,  iron-ores,  and  apatite,  the  quartz  and  orthoclase  some- 
times shewing  a  micrographic  intergrowth.  In  some  of  the 
granites,  graduating  into  granophyres,  of  Mull  and  the  Red 
Hills  of  Skye  augite  is  the  dominant  coloured  mineral,  but  it 
tends  to  be  converted  to  hornblende,  and  primary  hornblende 
often  accompanies  it. 

Augite-granites  with  anorthoclase  as  the  dominant  felspar 
('  soda-granites ')  are  described  from  Minnesota5,  New  Bruns- 
wick6, and  other  parts  of  North  America.  These  rocks  also 
tend  strongly  to  micrographic  structures,  and  graduate  into 
typical  granophyres. 

In  Southern  India  a  peculiar  hypersthene-granite  is  of 
wide-spread  occurrence,  and  has  been  described  by  Mr  Holland7 
under  the  name  charnockite.  The  typical  rock  consists  of 
quartz  and  potash-felspar,  with  oligoclase,  hypersthene,  opaque 
iron-ore,  and  a  little  zircon.  The  dominant  felspar  seems  to 
be  microcline,  often  with  parallel  microperthitic  intergrowths 
of  plagioclase.  The  rock  often  shews  some  gneissic  banding. 

1  Iddings,  in  Diller,  179,  180. 

2  Washington,  A.  J.  S.  (1898)  vi,  180,  181. 

3  Hawes,  A.  J.  S.  (1881)  xxi,  23. 

4  Teall,  pi.  xxxix,  fig.  2,  and  G.  M.  1885,  112-116 ;  Kynaston,  Tr. 
Edin.  G.  S.  (1899)  vii,  390-397. 

5  Grant,  2 1st  Ann.  Rep.  Geol.  Sur.  Minn.  (1894)  and  Arner.   Geol. 
(1893)  xi,  383-388. 

6  Mathew,  Tr.  N.  Y.  Acad.  Sci.  (1895)  xiv,  204-208,  pi.  xvi,  xvn. 

7  Mem.  Geol.  Stir.  Ind.  (1900)  xxviii,  134-141. 


40  APLITES:  PEGMATITES. 

Closely  related  to  the  granites  is  the  rock  known  as  aplite 
(granite-aplite).  It  occurs  as  veins  in  granite,  but  cutting  the 
latter  and  traversing  adjacent  rocks,  and  by  some  petrologists 
it  would  be  placed  in  the  hypabyssal  division.  It  is  a  fine- 
textured  rock  with  *  panidiomorphic '  to  granulitic  structure1 
and  is  somewhat  more  acid  than  the  associated  granite.  A 
characteristic  type  occurs  in  connection  with  the  muscovite- 
granites  near  Dublin  (Dalkey  and  Killiney).  It  consists  of 
microcline  with  some  oligoclase,  quartz,  muscovite,  and  red 
garnet.  An  aplite  at  Meldon  in  Devonshire2  is  of  similar 
character,  but  instead  of  garnet  contains  topaz  and  some 
colourless  or  pale  tourmaline.  The  Crosby  dyke3  in  the  Isle 
of  Man  may  be  referred  here.  It  consists  essentially  of  a 
granular  mosaic  of  clear  felspars,  quartz,  and  white  mica,  the 
dominant  felspar  being  an  albite.  Besides  the  abundant 
small  flakes  of  white  mica,  some  larger  hexagonal  plates  occur, 
and  sometimes  scattered  quartz-grains  or  larger  felspars. 
There  are  also  a  few  garnets  of  very  irregular  shapes,  giving 
a  sponge-like  appearance  in  section. 

Washington4  has  described  aplite  dykes  cutting  the  granite 
of  Essex  Co.,  Mass.,  and  Pirsson  notes  aplites  on  Coanicut 
Island,  R.I.  In  the  Sierra  Nevada  region  Turner5  has  remarked 
dykes  of  soda-aplite,  consisting  essentially  of  albite  and  quartz 
with  sometimes  muscovite,  besides  other  aplites  in  which  a 
potash-felspar  is  the  dominant  one. 

Many  of  the  rocks  termed  granulites  by  German  writers 
doubtless  belong  here.  They  will  be  noticed  in  a  later  chapter 
(Chap.  XXIL). 

The  pegmatites  belonging  to  this  family  of  rocks  (granite- 
pegmatites;  consist  essentially  of  microcline  or  orthoclase  and 
quartz,  often  with  white  mica  and  sometimes  red  garnet.  The 
texture  is  often  extremely  coarse,  and  there  is  a  frequent 
tendency  to  the  graphic  structure.  Such  rocks  are  extensively 
developed  in  connection  with  the  Archaean  gneiss  of  Sutherland. 

1  For  plate  of  aplite  from  near  Heidelberg  see  Berwerth,  Lief.  n. 

2  Teall,  p.  316  ;  McMahon,  G.  M.  1901,  316-319. 

3  Hobson,  Q.  J.  G.  S.  (1891)  xlvii,  440. 

4  Journ.  Geol.  (1899)  vii,  105,  106. 

5  Ibid.  156-158, 


TOURMALINE-GRANITES.  41 

Others  occur  in  Forfarshire l :  these  are  rich  in  muscovite, 
and  locally  carry  garnet  or  tourmaline.  It  may  be  observed 
that  these  British  pegmatites  are  not  rich  in  rare  or  special 
minerals.  In  the  United  States,  on  the  other  hand,  many  of 
the  most  noted  mineral- localities  are  furnished  by  pegmatites 
of  this  kind ;  e.g.  Stoneham  and  Hebron  in  Maine,  Chesterfield 
in  Massachussetts,  Haddam  in  Connecticut,  Pike's  Peak  in 
Colorado,  and  Harney's  Peak  in  the  Black  Hills  of  Dakota. 
Central  Maryland  is  another  district2.  Pegmatitic  and  aplitic 
dykes,  both  carrying  red  garnet,  occur  in  the  Montara  granite 
of  San  Francisco3,  and  such  dykes,  with  only  a  small  quantity 
of  mica,  are  associated  with  the  Santa  Lucia  granite  near 
Monterey4. 

The  tourmaline-granites  appear  as  modifications  of  more 
normal  granitic  rocks.  The  tourmaline  seems  to  take  the 
place  of  the  mica.  As  a  further  modification,  the  felspars 
may  be  replaced  partly  or  wholly  by  tourmaline  and  quartz, 
the  former  sometimes  occurring  in  little  needles  with  radiate 
grouping  imbedded  in  clear  quartz.  The  extreme  modification 
is  a  tourmaline-quartz-rock  or  schorl-rock,  in  which  felspar  is 
wholly  wanting,  while  tourmaline  may  occur  in  two  or  more 
habits,  as  crystals  or  grains  and  as  groups  of  needles.  All 
these  types  are  illustrated  among  the  Cornish5  and  Dartmoor 
granites.  A  curious  variety  known  as  luxulyanite  has  been 
described  by  Prof.  Bonney6.  Here  the  conversion  of  felspars 
into  clear  quartz,  crowded  with  radiate  groups  of  tourmaline 
needles,  can  be  traced  in  various  stages,  the  little  needles, 
about  '03  inch  in  length,  giving  pale  brown  and  light  indigo 
colours  for  longitudinal  and  transverse  vibrations  respectively, 
while  a  brown  tourmaline  in  distinct  grains  has  been  supposed 
to  represent  the  mica  of  the  granite.  A  rock  from  Trowles- 
worthy  Tor  shews  a  similar  replacement  of  felspar  (fig.  8,  A\ 

1  Barrow,  G.  M.  1892,  64 ;  Q.  J.  G.  S.  (1893)  xlix,  332-336. 

2  G.  H.  Williams,  loth  Ann.  Rep.  U.  S.  Geol.  Sur.  (1895)  675-684. 

3  Lawson,  ibid.  413. 

4  Lawson,  Bull.  Dep.  Geol.  Univ.  Gal.  (1893)  i,  16,  17. 

5  For  coloured  figure  of  tourmaline-granite  from  Cornwall  see  Cohen 
(3),  pi.  xxn,  fig.  2. 

6  M.  M.  (1877)  i,  215-222;   Semmons,  Pr.  Liverp.  G.  S.  (1878)  iii, 
357,  358. 


42 


GREISENS. 


and  has  in  addition  irregular  patches  of  isotropic  fluor  also 
enclosing  needles  of  tourmaline1. 

The  rock  known  as  greisen  (hyalomicte  of  French  writers) 
consists  essentially  of  quartz  and  white  mica,  which  seems  to 
be  often  a  lithia-bearing  variety.  The  Cornish  greisens2  are 
apparently  a  modification  of  the  granite  in  the  same  sense  as 
the  tourmaline-rocks  are,  but  with  a  different  result.  The 
place  of  the  felspar  is  taken  by  mica  and  topaz,  though 
tourmaline  is  also  met  with.  It  may  be  remarked  that  the 
topaz-rocks  of  Schneckenstein  and  Geyer  in  Saxony  are  closely 
allied  to  greisen.  Greisen  is  also  found  in  connection  with  the 
granite  of  the  Scilly  Isles.  In  Grainsgill,  Cumberland3,  it  has 
been  formed  at  the  expense  of  a  pegmatitic  modification  of 
the  Skiddaw  granite,  and  the  successive  stages  of  the  trans- 
formation can  be  studied.  The  white  mica  builds  sometimes 


B 


FIG.  8.     MODIFICATIONS  OF  GKANITE  ;    x  20. 

A.  Replacement  of  felspar  by  clear  quartz  full  of  tourmaline-needles, 
Trowlesworthy  Tor,  Cornwall :  with  remains  of  much-decomposed  felspar 
[1361].  B.  Greisen,  Grainsgill,  Cumberland :  consisting  of  quartz 
and  muscovite  with  only  occasional  relics  of  turbid  felspar  [1547]. 

1  Worth,  Trans.  Roy.  Geol.  Soc.  Cornw.  (1884)  x,  177-188. 

2  Teall,  315. 

3  Q.  J.  G.  S.  (1895)  li,  141. 


BASIC  SECRETIONS  IN  GRANITES.  43 

rather  large  flakes  (fig.  8,  B),  sometimes  aggregates  of  small 
scales,  and  in  both  cases  is  embraced  or  enclosed  by  a 
moderately  coarse  mosaic  of  clear  quartz.  An  American 
locality  for  typical  greisen  is  Hill  City  in  South  Dakota. 

In  conclusion  we  will  note  some  examples  of  the  dark,  fine- 
grained, ovoid  patches  frequently  enclosed  in  granitic  rocks, 
and  regarded  as  basic  secretions  separated  out  from  the  granite- 
magma  at  an  early  stage,  not  necessarily  in  situ.  Mr  J.  A. 
Phillips'  described  such  patches  from  the  muscovite-granites 
of  Gready  in  Cornwall  and  Foggen  Tor  on  Dartmoor  and  the 
biotite-granites  of  Shap  and  Peterhead,  and  he  distinguished 
them  from  foreign  fragments  caught  up  and  metamorphosed 
by  the  magma.  The  characteristic  of  the  true  secretions  is 
that  they  consist  of  the  same  minerals  as  the  enveloping  rock, 
but  contain  the  earliest  products  of  crystallization — such  as 
apatite,  magnetite,  and  sphene — in  larger  proportions,  and  are 
also  richer  in  the  ferro-magnesian  relatively  to  the  felspathic 
elements  of  the  rock.  Sometimes,  as  in  the  Criffel  granite2, 
we  may  observe  that  hornblende  is  more  plentiful  as  compared 
with  biotite  than  in  the  normal  rock,  and  similarly  plagioclase 
felspar  is  more  abundant  relatively  to  orthoclase.  The  numerous 
dark  patches  in  the  Shap  granite3,  rich  in  sphene  and  biotite 
(fig.  7,  B\  enclose,  like  the  normal  rock,  large  porphyritic 
crystals  of  orthoclase ;  but  these  are  partially  rounded  and 
corroded,  the  margin  of  each  crystal  being  replaced  by  plagio- 
clase and  quartz. 

Among  American  rocks  good  illustrations  are  afforded  by 
the  hornblende-granite  of  the  Wahsatch  Range  (Little  Cotton- 
wood  Canon,  Utah),  that  of  Essex  County,  Mass.4,  and  the 
biotite-granite  of  Mount  Ascutney,  Vt5. 

1  Q.  J.  G.  S.  (I860)  xxxvi,  1-21 ;  (1882)  xxxviii,  216,  217. 

2  Teall,  Mem.  Geol.  Sur.  Scot.,  Expl.  of  Sfieet  5  (1896)  42. 

3  Q.  J.  G.  S.  (1891)  xlvii,  281,  282,  pi.  xi,  fig.  2. 

4  Washington,  Journ.  Geol.  (1898)  vi,  795. 

5  Jaggar,  Bull.  148  U.  S.  G.  S.  (1897)  68. 


CHAPTER  III. 

SYENITES   (including  NEPHELINE-SYENITES). 

THE  syenites  are  even-grained,  liolocrystalline  rocks  con- 
sisting essentially  of  alkali-felspars,  and  in  one  group  fel- 
spathoid  minerals,  with  ferro-magnesian  constituents,  typically 
in  smaller  proportion,  and  various  minor  accessories.  The 
texture  is  often  rather  coarse  to  medium-grained,  and  the 
structure  is  that  characteristic  of  plutonic  rocks,  the  several 
minerals  following  the  normal  order  of  crystallization,  and  most 
of  them  having  only  imperfect  crystal  outlines  (hypidiomorphic 
structure  of  Rosenbusch).  In  many  syenites,  however,  the 
order  of  crystallization  is  modified  by  simultaneous  intergrowths 
of  different  minerals. 

This  family  of  rocks  is  less  widely  distributed  and  less 
abundant  than  the  granites.  Considered  from  a  chemical 
point  of  view,  it  is  characterized  by  an  unusually  high  per- 
centage of  alkalies.  In  the  syenites  which  depart  farthest  in 
this  respect  from  the  commoner  types  of  igneous  rocks,  the 
character  shews  itself  in  the  presence  of  felspathoid  con- 
stituents and  soda-bearing  ferro-magnesian  minerals. 

The  type  characterized  by  hornblende  and  alkali -felspars 
is  known  as  'syenite  proper'1,  or,  for  clearness,  hornblende- 
syenite.  When  biotite  more  or  less  completely  takes  the  place 
of  hornblende,  we  have  mica-syenite ;  and  when  augite  occurs 
prominently,  often  in  company  with  one  or  both  of  the  other 

1  The  original  syenite  of  Werner  was  the  hornblende-granite  of  Syene 
or  Assouan  on  the  Nile.  The  name,  however,  has  come  to  be  universally 
applied  to  the  family  under  notice,  rocks  often  hornblendic  but  typically 
free  from  quartz. 


FELSPARS   AND   NEPHELINE   OF   SYENITES.  45 

coloured  minerals,  augite-syenite.  The  group  characterized  by 
the  occurrence  of  nepheline  or  sodalite  in  addition  to  felspar  is 
named  nepheline-syenite,  or  often  elseolite-syenite,  without  dis- 
tinction according  to  the  dominant  ferro-magnesian  constituent, 
though  several  types,  mostly  of  restricted  occurrence,  have 
received  special  names.  A  leucite-syenite  is  known  only  in 
the  form  of  rocks  with  pseudomorphs  of  orthoclase,  elaeolite, 
muscovite,  etc.,  in  the  shape  of  leucite  (fig.  11). 

The  occurrence  of  subordinate  quartz  in  some  syenites 
gives  rise  to  the  varieties  quartz-syenite,  quartz-mica-syenite, 
and  quartz-aufjite-syenite,  but  free  silica  never  occurs  in  the 
nepheline-bearing  group.  On  the  other  hand  the  coming  in 
of  a  lime-soda-felspar  as  a  prominent  constituent  in  addition 
to  the  alkali-felspar  gives  rise  to  types  intermediate  between 
true  syenites  and  diorites,  and  to  these  the  name  monzonite  is 
sometimes  given. 

Constituent  minerals.  In  mode  of  occurrence,  in- 
clusions, alteration-products,  etc.,  the  felspars  of  syenites 
resemble  those  of  granites.  Besides  arthoclase,  microcline, 
and  albite  or  oligoclaxe,  there  occur,  especially  in  the  augite- 
and  nepheline-syenites,  felspars  rich  in  both  potash  and  soda, 
known  as  soda-orthoclase,  soda-microcline,  anorthoclase,  etc. 
These  are  regarded  by  some  mineralogists  as  intergrowths 
on  an  ultra-microscopic  scale  of  a  potash-  and  a  soda-felspar 
(cryptopertkite).  An  evident  parallel  intergrowth  of  albite 
and  microcline  or  albite  and  orthoclase  (microperthite)  is  also 
frequent  in  the  same  rocks. 

When  nepheline  occurs,  it  is  of  the  variety  known  as 
elwolite,  in  larger  and  less  perfect  crystals  than  the  nepheline 
of  volcanic  rocks.  If  idiomorphic,  it  forms  hexagonal  prisms 
with  the  basal  plane  bevelled  by  narrow  pyramid-faces.  In 
more  shapeless  crystals  the  straight  extinction  can  be  verified 
by  reference  to  rows  of  inclusions  which  follow  the  direction 
of  the  vertical  axis,  and  seem  to  determine  the  alteration  of 
the  mineral.  The  elseolite  is  colourless  or  often  rather  turbid. 
It  gives  rise  by  decomposition  to  various  soda-zeolites  or  to 
moderately  brightly  polarizing  prisms,  fibres,  and  aggregates  of 
cancrinite.  A  frequent  associate  of  elseolite  is  sodalite,  in 
dodecahedra  or  in  allotriomorphic  crystal-plates  and  wedges. 


46  DARK   MINERALS   OF  SYENITES. 

It  is  colourless  or  faint  blue  in  slices,  and  is  easily  recognized 
by  its  isotropic  behaviour.  It  encloses  fluid-pores,  microlites 
of  segirine,  etc.,  and  secondary  products  similar  to  those  of 
elaeolite. 

The  common  hornblende  of  syenites  is  partly  idiomorphic 
but  without  terminal  planes.  It  is  of  the  green  pleochroic 
variety,  giving  in  vertical  sections  a  maximum  extinction- 
angle  of  12°  to  16°.  Its  inclusions  and  alteration-products 
are  the  same  as  in  granite.  Some  augite-syenites  contain  the 
soda-amphibole  barkemcite  with  intense  brown  absorption  and 
pleochroism  and  an  extinction-angle  of  about  12°. 

The  augite,  when  it  occurs  as  an  accessory,  is  colourless  or 
very  pale  green,  with  the  same  properties  as  in  granite.  In 
the  augite-syenites  it  is  sometimes  pale  green  with  faint 
pleochroism,  sometimes  pale  brown  to  violet-brown  with  very 
distinct  pleochroism.  Various  types  of  schiller-  and  diallage- 
structures  are  sometimes  seen,  and  may  affect  only  a  portion — 
usually  the  interior — of  a  crystal  (fig.  10).  A  green  pleochroic 
wgirine  occurs  in  some  augite-syenites  and  many  nepheline- 
syenites,  and  intergrowths  of  this  with  augite  are  not 
uncommon. 

The  biotite  of  the  syenites  is  deep  brown,  becoming  green 
only  by  secondary  changes.  In  some  augite-  and  nepheline- 
syenites  vibrations  parallel  to  the  cleavage-traces  are  almost 
completely  absorbed.  The  mineral  is  roughly  idiomorphic, 
except  when  intergrown  with  hornblende  or  augite. 

When  quartz  occurs,  it  has  the  same  characters  as  in 
granite,  but  is  never  very  abundant.  It  does  not  occur  in 
the  nepheline-syenites  and  their  allies.  Most  syenites  contain 
plenty  of  sphene  in  good  crystals  shewing  the  cleavages  and 
often  the  characteristic  twinning1.  Zircon  is  common  in  small 
prisms  with  pyramidal  terminations,  as  in  the  granites.  In 
some  of  the  augite-syenites,  however,  it  builds  large  crystals 
of  simple  pyramidal  form.  It  is  easily  identified  by  its 
limpid  appearance  and  extremely  high  refringence  and  bire- 
fringence. Apatite  in  colourless  needles  is  widely  distributed 
in  syenites.  The  iron-ores  are  variable  in  quantity  :  they 

1  See  Rosenbusch-Iddings,  pi.  xi,  fig.  3;  xxm,  fig.  1:  Cohen  (3), 
pi.  xxx,  fig.  1. 


STRUCTURE   OF   SYENITES.  47 

include  magnetite,  ilmenite,  and  haematite,  the  last  two  often  in 
thin  flakes  enclosed  in  the  felspars.  An  occasional  accessory  is 
perofskite  in  small  octahedra,  distinguished  by  their  very  high 
refractive  index  and  feeble  double  refraction.  Special  types 
contain  melanite  garnet,  brown  in  slices  and  always  isotropic. 

Structure.  The  texture  of  the  syenites  and  the  mutual 
relations  of  their  constituent  minerals  are  normally  similar 
to  those  observed  in  the  granites,  Rosenbusch's  'order  of 
consolidation'  being,  as  a  rule,  followed.  In  the  typical 
hornblende-syenites  there  are  few  peculiarities.  When  quartz 
enters,  it  may  be  intergrown  in  micrographic  fashion  with 
part  of  the  orthoclase,  and  this  is  specially  the  case  in  some 
augite-syenites.  When  plagioclase  felspar  is  abundant,  it  is 
sometimes  moulded  by  shapeless  plates  of  orthoclase,  and  in 
the  same  rocks  reversals  of  order  between  the  bisilicates  and 
the  felspars  may  often  be  noticed. 

Where  the  felspathoids  occur,  their  place  in  the  order  of 
crystallization  is  a  variable  one.  These  minerals  usually  precede 
the  felspars,  but  may  continue  to  crystallize  to  a  later  stage. 
The  nepheline-syenites  not  infrequently  take  on  a  porphyritic 
character:  often  too  a  'trachytic'  structure,  marked  by  a 
partial  parallelism  of  felspars  with  tabular  habit. 

Some  syenites  contain  basic  secretions,  acid  veins,  pegmatite 
fringes  and  other  peculiarities  noticed  under  the  granites. 
Parallel  and  gneissic  structures  sometimes  come  in  locally 
(e.g.  Plauen'scher  Grund). 

Leading  types.  Although  typical  hornblende-syenites 
occur  in  this  country  (e.g.  Malvern),  very  little  has  been 
written  about  them,  and  for  the  type-rocks  we  must  go  to 
foreign  occurrences.  The  name  '  syenite '  as  found  in  many 
of  the  earlier  writings  and  maps  in  this  country  is  to  be  under- 
stood in  the  old  sense  of  hornblende-granite  (including  also 
granophyre,  etc.}  and  the  identification  of  hornblende  is  in  very 
many  cases  erroneous.  For  example,  the  so-called  'syenites'  of 
St  David's,  of  Ennerdale,  of  Carrock  Fell,  etc.,  have  no  claim  to 
the  title,  whether  the  word  be  used  in  its  original  or  its  modern 
sense. 

The  rock  taken  as  the  type  of  hornblende-syenite  is  that 
of  Plauen'scher  Grund  near  Dresden  (fig.  9).  It  is  composed 


48 


HORNBLENDE-SYENITES. 


essentially  of  orthoclase,  with  only  subordinate  oligoclase,  and 
green  hornblende.  Apatite,  magnetite,  and  sphene  occur  as 
accessories,  and  in  places  a  little  quartz.  There  is  a  variety 
in  which  biotite  occurs  in  addition  to  the  hornblende.  The 
rock  encloses  dark  basic  secretions  richer  in  plagioclase,  horn- 
blende, apatite,  magnetite,  and  sphene.  Further  there  are 
pegmatoid  acid  veins  of  coarse  texture,  in  which  the  more 
basic  minerals  occur  only  sparingly,  while  quartz  is  plentiful. 
Almost  the  same  description  applies  to  other  Saxon  syenites, 
such  as  that  of  Meissen,  which,  however,  has  rather  more 
oligoclase  and  brown  mica,  and  further  contains  a  little  more 
quartz,  either  in  grains  or  in  micrographic  intergrowth. 


ol 


FIG.  9.    HORNBLENDE-SYENITE,  PLAUEN'SCHER  GRUND,  DRESDEN  ;    x  20. 

Shewing  hornblende  (7t),  orthoclase  (or),  subordinate  oligoclase  (ol), 
sphene  (sp),  and  apatite  (ap)  [47]. 

A  syenite  like  that  of  Dresden,  but  sometimes  rich  in 
biotite,  occurs  near  Salem,  Mass.1  More  felspathic  varieties 
have  been  noted  from  Curtis  Point,  Beverley,  Mass.2  (with 
arfvedsonite-like  hornblende)  and  Albany,  N.H.  (with  blue 
riebeckite). 

1  Wadsworth,  G.  M.  1885,  207. 

2  Sears,  Bull.  Essex  Inst.  (1891)  xxiii. 


QUARTZ-SYENITES.  49 

Such  rocks  as  that  of  Meissen  may  with  propriety  be 
termed  quartz-syenite  (quartz-hornblende-syenite),  and  form 
a  connecting  link  with  the  hornblende-granites.  Again,  when 
a  triclinic  felspar  becomes  predominant  we  have  transitions 
to  quartz-diorite  (e.g.  Weinheim,  in  the  Odenwald,  near 
Heidelberg).  Brogger's  red  quartz-syenite  (Nordmark  type) 
from  the  Christiania  district  also  has  oligoclase  in  addition  to 
the  dominant  orthoclase,  and  sometimes  a  microperthitic 
intergrowth  of  albite  and  orthoclase.  Biotite  and  hornblende 
are  the  chief  ferro-magnesian  constituents,  but  green  augite 
and  aegirine  also  occur.  This  type  is  known  in  America,  e.g. 
in  the  Montreal  district1.  In  Sutherland  the  large  intrusive 
mass  of  Cnoc  na  Sr6ine2,  near  Loch  Borolan  consists  mainly  of 
a  quartz-syenite  approximating  to  the  Nordmark  type,  but  it 
graduates  on  the  one  hand  into  granite  and  on  the  other  into 
quartzless  syenite  and  other  more  remarkable  types. 

The  mica-syenite  type,  in  which  biotite  predominates  over 
hornblende,  is  of  uncommon  occurrence,  except  as  a  local 
variety  of  hornblende-syenite.  More  often  there  is  some 
quartz  present,  and  such  rocks  are  found  graduating  into 
biotite-granite.  Rosenbusch  mentions  mica-syenites  from 
Canada;  one  from  Star  Hill  Mine,  Portland  West,  P.  Q.,  rich 
in  apatite ;  another  from  Blessington  Mine,  Inchinbrooke, 
P.  0.,  with  some  augite  as  well  as  mica.  These  rocks  are  free 
from  quartz  or  plagioclase. 

Among  quartz-augite-syenites  may  be  mentioned  Brogger's 
Aker  type  from  the  Christiania  district,  which  contains  plenty 
of  plagioclase  as  well  as  orthoclase,  and  has  resemblances  to 
the  Monzoni  rocks.  Biotite  occurs  in  addition  to  the  pale 
green  augite.  A  similar,  but  rather  more  acid,  rock  from 
Essex  County,  Mass.,  has  primary  hornblende  in  addition 
to  augite3. 

Other  quartz-syenites  characterized  by  augite  shew  a  strong 
tendency  to  micrographic  intergrowth  of  quartz  and  felspar. 
This  is  seen  in  the  larger  pre-Carboniferous  intrusions  of 

1  Dresser,  Amer.  Geol  (1901)  xxviii,  207,  208  (Shefford  Mt.). 

2  Teall,  G.  M.  1900,  385-392. 

3  Washington,  Journ.  Geol.  (1898)  vi,  797. 

H.  P.  4 


50  MONZONITES. 

Leicestershire  (excepting  the  Mount  Sorrel  granite),  which 
indeed  may  be  classed  as  a  less  acid  type  of  granophyre.  The 
augite  tends  to  pass  into  uralitic  hornblende,  and  epidote  is  a 
characteristic  secondary  product  in  the  rocks.  Examples  are 
seen  at  Groby,  Bradgate  Park,  Markfield,  and  Garendon,  all 
in  the  Charnwood  Forest  district1. 

A  special  type  of  augite-syenite  is  presented  by  the  Triassic 
intrusions  of  Monzoni  in  the  southern  Tirol  (monzonite2  of  De 
Lapparent),  which  are  associated  with  diabases  and  other  basic 
rocks.  Orthoclase  is  sometimes  the  only  felspar,  but  usually 
there  is  a  plagioclase  in  addition,  forming  idiomorphic  crystals 
enclosed  witl^the  other  minerals  by  plates  of  orthoclase.  The 
augite  often  passes  over  into  green  hornblende,  but  the  latter 
mineral  also  occurs  as  an  original  constituent.  Biotite  is 
usually  present,  in  flakes  sometimes  earlier,  sometimes  later, 
than  the  plagioclase.  Sphene  is  frequent,  and  zircon  is  often 
enclosed  by  the  mica.  Other  constituents  are  apatite,  mag- 
netite, and  pyrites,  and  in  some  varieties  a  little  interstitial 
quartz. 

In  America  Weed  and  Pirsson  have  described  a  rock 
closely  resembling  the  typical  monzonites  from  Yogo  Peak, 
Montana3.  This  rock,  with  about  equal  amounts  of  felspar 
and  augite,  graduates  on  the  one  hand  into  a  more  felspathic 
augite-syenite  and  on  the  other  into  a  thoroughly  basic  type 
very  rich  in  augite.  This  last  (Shonkin  type)  was  first 
distinguished  by  the  same  writers  at  Square  Butte  in  the 
Highwood  Mts,  Mont.4  It  consists  of  predominant  augite 
with  orthoclase,  albite,  and  anorthoclase,  apatite,  biotite, 
olivine,  etc.,  and  may  be  compared  with  the  basic  modifi- 
cations of  the  rocks  of  Monzoni  ('  pyroxenites '  of  Brogger). 

1  Bonney,  Q.  J.  G.  S.  (1878)  xxxiv,  214-218. 

2  Brogger  makes  a  distinct  family  of  monzonites,  characterized  by 
the  occurrence  of  orthoclase  and  plagioclase   felspars  in   about  equal 
amounts,  and  including  more  and  less  acid  members. 

3  A.  J.  S.  (1895)  1,  467-479;  ^th  Ann.  Eep.  U.  S.  Geol.  Sur.  (1900) 
part  in,  475-479.   For  another  American  occurrence  see  Tower  and  Smith, 
19th  Ann.  Eep.  U.  S.  Geol.  Sur.  (1899)  part  m,  645,  646  (Tintic  Mts,  Utah). 

4  Bull.  Geol.  Soc.  Amer.  (1895)  vi,  408-415 ;  cf.  20th  Ann.  Rep.  U.  S. 
Geol.  Sur.  (1900)  part  in,  479-484,  pi.  LXXII. 


AUGITE-SYENITES.  51 

From  Magnet  Cove,  Arkansas,  Washington1  describes  a  similar 
but  nepheline-bearing  rock  under  the  name  '  covite.' 

A  remarkable  basic  rock,  comparable  with  Brogger's  olivine- 
monzonite,  occurs  at  Kentallen  and  other  places  in  Argyllshire 
(Kentallen  type)2.  It  consists  of  olivine,  pale  green  augite, 
plagioclase,  and  interstitial  biotite  and  orthoclase.  It  shews 
considerable  variation,  sometimes  approximating  to  the  Shonkin 
type. 

A  peculiar  augite-syenite  (Laurvig  type),  allied  in  some 
respects  to  the  nepheline-syenites,  occurs  among  the  Devonian 
intrusions  of  the  Christiania  district.  While  augite  is  usually 
the  dominant  ferro-magnesian  element,  it  is  often  accompanied 
by  biotite,  segirine,  hornblende,  or  arfvedsonite,  and  the  rock 
thus  passes  into  mica-syenite,  etc.  Alkali -felspars  (orthoclase, 
microcline,  albite,  cryptoperthite,  etc.)  make  up  the  bulk  of 
the  rock,  and  are  often  intergrown  with  one  another.  Not 
infrequently  they  have  a  schiller-structure.  A  little  quartz  is 
rarely  present;  on  the  other  hand  elreolite  and  sometimes 
olivine  may  occur  as  minor  accessories.  The  augite  is  oc- 
casionally green,  but  commonly  light  brown  with  a  violet  tone 
and  slight  pleochroism :  schiller-structure  is  common.  The 
hornblende  is  green  or  occasionally  brown,  the  biotite  a  very 
deep  brown.  The  latter  mineral  is  roughly  idiomorphic, 
except  when  it  is  massed  round  magnetite  or  forms  a  marginal 
intergrowth  with  augite.  The  iron-ores  are  magnetite  and 
sometimes  haematite :  apatite  is  universal,  but  sphene  is  typi- 
cally absent.  Zircon  is  a  constant  accessory,  and  sometimes 
builds  large  crystals,  giving  the  variety  '  zircon-syenite '  of 
von  Buch  and  other  early  writers.  These  augite-syenites  are 
common  as  boulders3  on  our  East  coast  (fig.  10). 

Other  augite-syenites  rich  in  microperthitic  intergrowths 
occur  in  New  Hampshire  (Jackson,  Stark,  and  Columbia)  and 
in  the  Sawtooth  Mts  of  Texas.  Examples  having  regirine- 
augite  as  the  dominant  coloured  silicate  come  from  the 

1  Journ.  Geol.  (1901)  ix,  612-615. 

2  Teall,  pi.  xvi,  fig.   1,  and  Ann.  Rep.  Geol.  Sur.  for  1896,  22,  23; 
Hill  and  Kynaston,  Q.  J.  G.  S.  (1900)  Ivi,  531-540,  pi.  xxix,  xxx;  Hill, 
Summary  of  Progress,  Geol.  Sur.  for  1899,  48-53. 

3  Proc.  Yorks.  Geol.  Pol.  Soc.  (1889-90)  xi,  303,  304,  410. 

4—2 


52  AUGITE-SYENITES. 

Bearpaw  Mts  in  Montana1  and  Mosquez  Canon,  Texas,  while 
a  more  typical  cegirine-syenite  is  recorded  from  Fourche  Mt., 
Ark. 


FlG.   10.       AUGITE-SYENITE    (LAUKVIG    TYPE)    FEOM   A    BOULDER   ON 

THE  YORKSHIRE  COAST  ;    x  20. 

The  minerals  seen  are  cryptoperthite  felspar  (/)  in  large  plates, 
augite  (a)  with  schiller-strueture  in  the  interior  of  the  crystal,  deep 
brown  biotite  (b),  magnetite  (m),  and  apatite  (ap)  [1841]. 

As  a  connecting  link  between  syenites  proper  and  nepheline- 
syenite  we  have  the  Pulaski  type  of  J.  F.  Williams2  from 
Fourche  Mt.,  near  Little  Rock,  Arkansas,  in  which  nepheline 
is  only  an  accessory  constituent.  Various  alkali-felspars 
occur,  soda-felspar  predominating,  and  various  ferro-magnesian 
minerals,  of  which  hornblende  is  the  chief.  A  neighbouring 
rock,  described  under  the  name  ela3olite-syenite,  seems  to  be 
not  essentially  different3.  The  same  type  occurs  associated 
with  nepheline-syenites  in  other  localities,  e.g.  near  Montreal4 

1  Weed  and  Pirsson,  A.J.S.  (1896)  ii,  136,  137. 

2  Ign.  Rocks  of  Arkansas,  vol.  ii  of  Ann.  Rep.  Geol.  Sur.  Ark.  for  1890, 
55-69  ;  Diller,  pp.  194-197. 

3  Ign.  Rocks  Ark.  74-81 ;  Washington,  Journ.  Geol.  (1901)  ix,  610. 

4  Dresser,  Amer.  Geol.  (1901)  xxviii,  209,  210  (Shefford  Mt.). 


NEPHELINE-SYENITES.  53 

and  in  Essex  County,  Mass.1  Another  American  rock,  from 
Red  Hill  in  New  Hampshire2,  is  referred  by  Rosenbusch  to 
the  Umptek  type  of  Ramsay  (from  the  Kola  peninsula  in 
Finland). 

Of  true  nepheline- syenites  a  good  example  is  the  coarse- 
grained Laurdal  type  of  the  Christiania  district.  It  differs 
from  the  neighbouring  Laurvig  rock  chiefly  in  the  presence  of 
abundant  nepheline  in  large  partly  idiomorphic  crystals  and 
often  sodalite.  There  is,  as  before,  considerable  variety  of 
alkali-felspars,  cryptpperthite  predominating.  The  ferro- 
magnesian  minerals  include  deep-brown  mica  and  either  a 
greenish  a3girine-augite  or  the  violet-brown  augite  noted  in 
the  other  rock.  Apatite  is  abundant. 

A  well-known  nepheline-syenite  is  that  of  Sierra  de 
Monchique  in  Portugal3  (Foya  type).  Here  the  felspar  is 
orthoclase  and  is  in  excess  of  the  nepheline  (elrcolite) ; 
sodalite  is  often  present ;  the  coloured  minerals  are  sub- 
ordinate hornblende,  augite  edged  with  rcgirine-augite,  and 
biotite ;  while  apatite,  magnetite,  and  abundant  sphene  are 
also  present.  Rocks  generally  comparable  with  this  occur  in 
Brazil,  near  Montreal  (with  melanite),  at  Salem  and  Marble- 
head4  (Mass.),  in  the  Crazy  Mts  (Mont.)5,  in  the  Cripple 
Creek  district  (Colo.)6,  at  Mt.  Ord  and  Paisano  Pass  (Tex.)7, 
and  at  several  localities  in  Arkansas8.  Some  of  the  Arkansas 
rocks  have  porphyritic  modifications.  At  Beemerville  (N.J.)9, 
again,  occurs  a  variety  with  very  large  crystals  of  orthoclase, 

1  Washington,  Journ.  Geol.  (1898)  vi,  804-807. 

2  Bay  ley,  Bull.  Geol,  Soc.  Amcr.  (1892)  iii,  245-253. 

3  Sheibner,  Q.  J.  G.  S.  (1879)  xxxv,  42-47,  pi.  n. 

4  Wadsworth,  Proc.  Bost.  Soc.  Nat.  Hist.  (1882)  xxi,  406  ;  G.  M.  1885, 
208,  209;  Sears,  Hull.  Essex  Inst.  (1893)  xxv;  Washington,  Journ.  Geol. 
(1898)  vi,  801-804. 

5  Wolff  and  Tarr,  Bull.  Mus.  Comp.  ZooL  Harv.  (1893)  xvi,  230,  231. 

6  Cross,  16th  Ann.  Rep.  U.  S.  Geol.  Sur.  (1895)  part  n,  43,  44. 

7  Osann,  4th  Ann.  Hep.  Geol.  Sur.  Tex.  (1893;  126-129. 

8  J.  F.  Williams,  Ign.  Rocks  Ark.  (1890)  85-87  (Fourche  Mt.) ;  132-139 
(Saline  Co.) ;  233-238  (Magnet  Cove) ;  349,  350  (Potash  Sulphur  Springs). 
On  the  Magnet  Cove  (Diamond  Jo)  rock  see  also  Washington,  Journ.  Geol. 
(1891)  ix,  610,  611. 

9  Emerson,  A.  J.  S.  (1882)  xxiii,  302-308;  Kemp,  Trans.  N.  Y.  Acad. 
Sci.  (1892)  xi,  63 ;  Iddings  in  Diller,  209,  210. 


54  NEPHELINE-SYENITES. 

the  interspaces  filled  by  .little  prisms  of  segirine  and  abundant 
elseolite,  partly  changed  to  cancrinite. 

Among  other  nepheline-syenites  may  be  mentioned  the 
Miask  type  from  the  Urals,  in  which  a  deep  brown  mica 
is  the  most  prominent  constituent,  plagioclase  is  abundant, 
frequently  intergrown  with  the  orthoclase,  and  zircon  is  a 
characteristic  accessory.  The  Ditro  type,  from  Transylvania, 
also  carries  mica,  but  much  less  plentifully :  it  is  distinguished 
by  its  abundance  of  allotriomorphic  sodalite  and  by  the 
variety  and  intimate  intergrowths  of  its  felspars,  which 
include  microcline  as  well  as  orthoclase  and  oligoclase.  Can- 
crinite, sphene,  zircon,  and  perofskite  also  occur1.  In  the 
Litchfield  type2,  from  Maine,  albite  constitutes  about  half 
of  the  rock,  the  other  minerals  being  orthoclase,  microcline, 
elseolite,  sodalite,  cancrinite,  a  deep  green  biotite  (lepido- 
inelane),  and  a  little  zircon.  A  variety  from  Dungannon3  in 
Ontario  resembles  the  Litchfield  rock  in  the  predominance  of 
a  soda-felspar,  but  is  richer  in  nepheline.  In  one  modification 
the  felspar  disappears,  and  the  rock  consists  merely  of  nephe- 
line with  a  little  hornblende  or  mica  (hornblende-ijolite) :  an 
interesting  feature  is  the  occurrence  of  corundum4.  The 
ijolite  of  Ramsay  and  Berghell,  from  Finland,  consists 
essentially  of  nepheline  and  augite,  usually  with  melanite 
(a  titaniferous  variety),  felspar  being  wholly  absent.  A  rock 
of  this  type  forms  part  of  the  curious  igneous  complex  of 
Magnet  Cove  in  Arkansas5. 

The  only  nepheline-syenite  known  in  Britain  occurs  in 
association  with  other  syenitic  rocks  on  Cnoc  na  Sr6ine  in 
Sutherland.  As  described  by  Mr  Teallfi,  it  consists  of 
nepheline  and  alkali-felspar  in  about  equal  amounts,  with 
some  greenish  biotite  and  melanite  garnet. 

1  For  coloured  figures  of  these  rocks  see  Fouque"  and  Levy,  pi.  XLV, 
fig.  1. 

2  Bayley,  Bull.  GeoL  Soc.  Amer.  (1892)  iii,  235-241,  and  in  Ciller, 
201-209,  pi.  xxix. 

3  Adams,  A.  J.  S.  (1894)  xlviii,  10-16. 

4  Coleman,  Rep.  Bur.  Mines,  Toronto  (1899)  viii,  part  n,  250-253. 

5  Washington,  Bull.  Geol.  Soc.  Amer.  (1900)  xi,  400  ;  Journ.  Geol. 
(1901)  ix,  618.     This  is  the  '  Ridge  type '  of  Williams,  Ign.  Rocks  Ark. 
(1890)  229-231. 

6  G.  M.  1900,  387,  388. 


LEUCITE-SYENITE.  55 

Associated  with  the  rock  just  mentioned  is  a  very  interest- 
ing one  which  may  be  styled  a  leucite-syenite  (Borolan  type)1. 
It  is  exposed  at  Loch  Borolan,  and  occurs  also  in  the  adjacent 
part  of  Ross.  The  rock  consists  essentially  of  orthoclase,  a 
brown  melanite  garnet,  and  a  green  or  green-brown  biotite. 
A  green  monoclinic  pyroxene  is  present  in  many  examples  : 


FIG.  11.     MELANITE-LEUCITE-SYENITE  (BOROLANITE),  LOCH  BOROLAN, 
SUTHERLAND  ;  x  5. 

Composed  essentially  of  orthoclase  and  the  iron-garnet  (melanite) 
with  some  pale  green  biotite.  The  clear  spaces  represent  pseudomorphs 
of  orthoclase  after  leucite  [2956]. 

a  brown  pleochroic  sphene,  apatite,  and  magnetite  occur  as 
accessories.  Much  of  the  orthoclase  occurs  in  the  form  of 
rounded  patches,  \  to  f  inch  in  diameter,  which  replace 
leucite  crystals  (fig.  11).  A  similar  rock,  differing  by  the 

1  Dakyns  and  Teall,  Tr.  Roy.  Soc.  Edin.  (1892)  xxxvii,  163-172,  with 
plate ;  Teall,  G.  M.  1900,  389. 


56  SYENITE-PEGMATITES. 

presence   of   abundant  nepheline,   is   found   in   the   igneous 
complex  of  Magnet  Cove,  Ark.1 

A  sodalite-syenite,  with  little  or  no  elseolite,  seems  to  be  an 
uncommon  type.  It  has  been  found  in  the  High  wood  Mts  of 
Montana2  and  at  Cotton  wood  Creek 3  in  the  same  state. 

Among  special  modifications  of  syenitic  rocks  may  be 
mentioned  the  syenite-aplites  and  syenite-pegmatites  described 
by  Brogger  as  associated  with  the  augite-  and  nepheline- 
syenites  of  the  Christiania  district.  The  pegmatites  are 
remarkable  not  only  for  the  frequent  perthitic  intergrowths  of 
potash-  and  soda-felspars,  but  also  for  graphic  intergrowths 
of  the  felspars  with  the  ferro-magnesian  minerals  and  with 
ela3olite  and  sodalite ;  and  they  are  famous  as  the  home  of 
many  rare  minerals.  Some  of  these  features  are  reproduced 
in  the  pegmatites  associated  with  the  Arkansas  nepheline- 
syenites4. 

1  J.  F.  Williams,  Ign.  Rocks  Ark.  (1890)  267-276 ;  Washington,  Pull. 
GeoL  Soc.  Amer.  (1900)  xi,  399,  and  Journ.  Geol.  (1901)  ix,  615-617.    The 
latter  author  gives  to  this  type  the  name  '  arkite.' 

2  Lindgren,  A.  J.  S.   (1893)  xlv,  290-297 ;  Weed  and  Pirsson,  Bull. 
GeoL  Soc.  Amer.  (1895)  vi,  416,  417. 

3  Merrill,  Pr.  U.  S.  Nat.  Mas.  (1894)  xvii,  671-673. 

4  J.  F.  Williams,  Ign.  Rocks  Ark.  (1890)  143-146,  238-258. 


CHAPTER  IV. 

DIORITES. 

THE  diorites  are  p] atonic  rocks  of  medium  to  coarse 
texture,  consisting  essentially  of  a  soda-lime  felspar  and  horn- 
blende, with  less  important  constituents.  The  family  so 
denned  cannot  be  regarded  as  a  natural  one,  its  members 
ranging  in  chemical  composition  from  sub-acid  to  thoroughly 
basic.  The  gabbros  (characterized  by  pyroxenes  in  place  of 
hornblende)  also  include  intermediate  as  well  as  basic  rocks, 
and  the  distinction  between  the  hornblende-  and  augite-bearing 
types  is  rather  an  artificial  one.  It  was  established  before 
the  strong  tendency  of  augite  to  pass  over  into  hornblende 
was  thoroughly  appreciated  :  later  research  has  shewn  the 
certainty  of  some,  and  the  possibility  of  many,  of  the  rocks 
that  have  been  termed  diorites  being  really  amphibolized 
pyroxenic  rocks. 

The  more  acid  diorites  contain  free  silica  (quartz-diorites), 
and,  except  for  the  smaller  proportion  of  quartz  and  the 
nature  of  the  felspars,  do  not  differ  much  from  the  hornblende- 
granites1.  They  may  have  biotite  in  addition  to  hornblende 
(qiiartz-mica-diorites),  or  in  some  cases  augite.  In  the  diorites 
proper,  without  quartz,  mica  is  not  common,  but  the  horn- 
blende may  be  accompanied  by  augite  or  sometimes  enstatite. 
The  hornblende  is  more  abundant  relatively  to  the  felspar 
than  in  the  preceding  types,  and  some  of  the  more  basic 
diorites  consist  chiefly  of  hornblende.  These  are  the  '  amphi- 
bolites'  of  some  authors2.  In  some  types  olivine  enters  as  a 
constituent  (plimne-diorites). 

1  See  Berwerth,  Lief.  1,  quartz-diorite  from  the  Vosges. 

2  For  a  hornblende-rock  (local  modification  of  a  diorite)  see  Fouqu6 
and  Michel  Levy,  pi.  xxm. 


58 


FELSPARS   OF   DIORITES. 


The  occurrence  of  felspathoid  minerals  in  dioritic  rocks 
seems  to  be  very  exceptional.  The  theralites  of  Rosenbusch 
may  be  regarded  as  nepheline-diorites  and  nepheline-gabbros, 
but  comparatively  little  is  yet  known  of  such  rocks. 


FIG.  12.     CRYSTALS  OF  PLAGIOCLASE  FELSPAR  IN  QUARTZ-MICA-DIORITE, 
BEINN  NEVIS  ;    x  20,  CROSSED  NICOLS. 

The  vibration -planes  of  the  nicols  are  indicated  by  the  lines  (ni). 
A  shews  the  association  of  twin-lamellation  on  the  albite  (a)  and 
pericline  (p)  laws.  B  shews  carlsbad  twinning  (c)  combined  with  albite- 
twin-lamellation  (a)  and  with  zonary  banding  [397]. 

Constituent  minerals.  The  felspar  of  the  diorites  is 
oligoclase,  andesine,  or  labradorite,  or  exceptionally  a  more 
basic  variety.  The  twin-lamellation  on  the  albite  type  is  often 
accompanied  by  pericline  or  carlsbad  twinning  (fig.  12,  A). 
In  the  quartz-diorites  especially,  the  crystals  frequently  shew 
between  crossed  nicols  a  marked  zonary  banding,  the  central 
and  marginal  portions  of  a  crystal  often  giving  widely  different 
extinction-angles,  and  the  successive  layers  growing  more  acid 
from  within  outwards  (fig.  12,  B).  In  natural  light  the  zones 
of  growth  may  be  indicated  by  the  disposition  of  fluid-pores, 
minute  scales  of  haematite,  or  other  inclusions.  The  crystals 
are  often  clouded  by  a  fine  dust  (probably  kaolin),  and  may 
also  furnish  by  their  alteration  scales  of  colourless  mica 


HORNBLENDE  OF   DIORITES. 


59 


(paragonite  ?),  grains  of  epidote,  calcite,  etc.  A  little  ortho- 
close  may  be  present  as  an  accessory,  behaving  in  the 
quartz-diorites  as  in  granites,  while  in  typical  diorites  it 
occurs  interstitially. 

The  hornblende,  when  idiomorphic,  shews  the  prism-faces 
and  usually  the  clinopinacoid,  and  terminal  planes  are  often 
present.  Twinning  is  common,  and  the  prismatic  cleavage  is 
always  well  pronounced.  In  the  quartz-diorites  the  mineral, 
usually  in  imperfect  crystals,  is  green,  as  in  granites ;  in  more 
normal  diorites  it  has  brownish-green  or  greenish -brown 
colours ;  and  in  the  most  basic  types  the  original  hornblende 
is  usually  of  some  greenish  shade  of  brown,  or  even  approaches 
the  deep  brown  of  *  basaltic  hornblende.'  Pale  colours  result 
from  bleaching,  or  are  found  in  secondary  outgrowths1  of  the 
brown  crystals,  and  these  are  green  rather  than  brown.  Two 
kinds  of  outgrowth  or  enlargement  of  hornblende  crystals  are 
to  be  observed  in  some  basic  diorites,  the  new  growth  being  in 


h 


FIG.  13. 


BASIC  DIORITE,  LLYS  EINION,  NEAR  LLANERCHYMEDD, 
ANGLESEY  ;    x  20. 


The  original  idiomorphic  brown  hornblende  has  an  extension  of 
green  hornblende  on  the  clinopinacoid  faces  (h)  and  also  a  secondary 
fibrous  outgrowth  on  the  terminal  planes  (h').  The  felspar  (/)  is  much 
decomposed,  and  crystalline  calcite  (c)  has  been  produced  [539]. 

1  Van  Hise,  A.J.S.  (1887)  xxxiii,  385-388,  with  figures. 


60  DARK    MINERALS   OF   DIORITES. 

both  cases  in  crystalline  continuity  with  the  old.  In  one  case 
a  growth  of  green  hornblende  takes  place  on  the  clinopinacoid 
faces  so  as  to  extend  the  crystal,  with  idiomorphic  contour, 
in  the  direction  of  the  orthodiagonal :  in  the  other  case  pale 
green  or  colourless  hornblende  grows  so  as  to  extend  a  crystal 
in  the  direction  of  its  length,  and  may  present  new  crystal- 
faces,  or  abut  on  another  crystal,  or  frequently  terminate  in  a 
ragged  fibrous  fringe.  The  second  type  of  outgrowth  at  least 
is  of  secondary  origin,  and  is  formed  at  the  expense  of  other 
minerals  (fig.  13).  Besides  more  usual  types  of  alteration1,  the 
brown  hornblende  of  diorites  may  shew  bleaching,  with  separa- 
tion of  magnetite,  or  it  may  be  converted  into  a  brown  mica 
or  into  green  blades  of  actinolite. 

The  deep  brown  biotite  of  the  diorites  occurs  in  idiomorphic 
flakes,  or  sometimes  intergrown  with  hornblende.  It  is  usually 
not  rich  in  inclusions.  It  becomes  green  only  by  partial 
decomposition. 

The  rhombic  pyroxene  found  in  a  few  diorites  is  a  variety 
poor  in  iron  (enstatite)  and  is  usually  converted  into  pseudo- 
morphous  pale  bastite. 

When  augite  is  present,  it  is  of  a  variety  sensibly  colour- 
less in  slices.  If  idiomorphic,  it  shews  the  octagonal  cross- 
section  due  to  equal  development  of  the  pinacoids  and 
prism-faces,  with  good  prismatic  cleavage  and  not  infrequently 
lamellar  twinning  parallel  to  the  orthopinacoid.  A  not 
uncommon  feature  in  diorites  is  a  parallel  growth  of  augite 
and  hornblende,  a  crystal -grain  of  the  former  mineral  con- 
stituting a  kernel,  round  which  a  shell  of  brown  hornblende 
has  grown,  and  this  seems  to  occur  specially  in  the  neighbour- 
hood of  grains  of  iron-ore.  This  must  be  distinguished  from 
another  phenomenon  frequent  in  the  augite-bearing  diorites, 
viz.  the  conversion  of  augite  into  brown  hornblende  as  a 
secondary  change2.  This  process  usually  begins  at  the  margin 
of  a  crystal  or  grain,  but  proceeds  irregularly,  shewing  a  very 
intricate  boundary  between  the  two  minerals  and  often  ragged 
scraps  of  one  enclosed  by  the  other.  When  the  conversion  is 

1  Zirkel,  Micro.  Petr.  Fortieth  Parallel,  pi.  in,  figs.  2,  3,  4. 

2  Q.  J.  G.  S.  (1888)  xliv,  452,  453 ;  M.  M.  (1888)  viii,  31-34. 


ACCESSORY   MINERALS   OF   DIORITES.  61 

complete,  the  secondary  hornblende  can  be  distinguished  from 
original  only  by  inference,  as,  e.g.  when  it  shews  the  external 
form  of  augite.  In  both  phenomena  the  augite  and  hornblende 
have  their  plane  of  symmetry  and  longitudinal  axis  in  common, 
and  in  longitudinal  sections  both  extinguish  on  the  same  side 
of  the  axis. 

The  quartz  of  quartz-diorites  has  the  same  general  charac- 
ters as  that  of  granites. 

The  olivine  which  occurs  in  some  basic  diorites  is  often  in 
rather  rounded  crystals  enclosed  by  the  hornblende.  It  is 
easily  recognised  by  its  high  refractive  index  and  very  strong 
double  refraction.  The  mineral  is  readily  altered  into  ser- 
pentine, carbonates,  and  especially  pale  fibrous  amphibole,  the 
last  often  grown  in  crystalline  continuity  with  adjacent 
original  hornblende. 

Among  the  iron-ores,  magnetite  is  the  most  usual,  but 
ilmenite  is  also  found.  Common  accessories  in  some  varieties 
are  zircon  and  sphene  in  characteristic  crystals.  Apatite  is 
general,  and  in  some  basic  diorites  abundant:  in  the  coarse- 
grained rocks  it  sometimes  builds  rather  large  prisms. 

Structure.  The  structure  of  the  dioritic  rocks  is 
variable.  In  the  quartz-diorites1  the  mutual  relations  of  the 
minerals  are  those  noticed  in  granites,  though  sometimes  a 
part  of  the  felspar  has  crystallized  before  the  ferro-magnesian 
minerals.  A  micrographic  intergrowth  of  quartz  and  felspar 
is  not  infrequent.  Many  of  the  quartzless  diorites  also  follow 
what  may  be  called  the  normal  order  of  crystallization. 
Rosenbusch  points  out  that  the  most  marked  pauses  in  the 
process  of  consolidation  have  occurred  before  the  separation  of 
the  ferro-magnesian  minerals  and  after  that  of  the  plagioclase; 
so  that  while  the  apatite,  sphene,  etc.,  and  the  plagioclase  may 
be  markedly  idiomorphic,  the  hornblende,  biotite,  and  augite 
tend  to  occur  in  much  more  irregularly  shaped  crystals. 
When  a  miarolitic  structure  results  from  the  tendency  to 
idiomorphism  in  the  latest  crystallized  elements,  it  is  com- 
monly obscured  by  the  cavities  becoming  filled  by  calcite  and 
other  secondary  products. 

1  Benverth,  Lief.  I  (Schwarzenberg,  Vosges). 


62  STRUCTURES   OF   DIORITES. 

A  different  type  of  structure,  though  connected  by  transi- 
tions with  the  preceding,  is  found  in  many  dioritic  rocks. 
Here  the  plagioclase  has  crystallized  earlier,  or  at  least  ceased 
to  crystallize  earlier,  than  the  bisilicates ;  so  that  the  dominant 
felspar  presents  idiomorphic  outlines  to  the  hornblende  and  (if 
present)  augite.  These  latter  may  wrap  round,  or  even 
enclose,  the  felspar  crystals,  giving  an  'ophitic'  structure 
identical  with  that  described  below  as  characteristic  of  the 
diabases,  and  the  hornblendic  rocks  exhibiting  this  character 
have  sometimes  been  termed  hornblende-diabases.  Such  a 
structure  is  found  more  or  less  markedly  in  many  of  the  more 
basic  diorites,  arid  is  especially  common  in  rocks  in  which  the 
hornblende  is  in  great  part  derivative  after  augite,  but  original 
hornblende  moulded  on  felspar  is  also  found. 

Pegmatoid  and  aplitic  structures  are  less  common  in  this 
family  than  in  the  granites  and  syenites. 

A  porphyritic  structure  is  not  common  in  true  diorites, 
but  may  come  in  as  a  marginal  modification  of  a  boss  or 
stock,  the  porphyritic  elements  being  crystals  of  hornblende 
or  felspar. 

As  a  more  special  type  of  structure  may  be  mentioned  the 
orbicular  (in  the  so-called  corsite  or  napoleonite),  where  the 
bulk  of  the  rock  consists  of  spheroidal  growths.  These  have 
a  radial  structure  and  consist  of  concentric  shells  composed 
essentially  of  hornblende  and  felspar  alternately.  A  well- 
known  rock  of  this  character  comes  from  San  Lucia  di  Tallano 
in  Corsica1. 

Leading  types.  The  quartz-diorite  of  the  Adamello 
Alps,  on  the  border  of  Italy  and  the  Tirol  (Tonale  type) 
comes  very  near  in  characters  to  some  granites 2,  and  has  also 

1  Cohen  (3),  pi.  LXXI,  fig.  3;  Robertson,  Tr.  G.  S.   Glasgow  (1883) 
vii,  210. 

2  This  is  the  'tonalite'  of  vom  Rath.     Since  it  is  an  extreme  type, 
and  is  classed  by  some  petrologists  with  the  granites,  it  is  confusing 
to  extend  this  name,  as  some  writers  have  done,  to   all   the  quartz- 
diorites.     Brogger  restricts  the  term  to  the  type  free  from  any  alkali- 
felspar;   that  with   both  an   alkali-  and  a  lime-soda-felspar  he   styles 
adamellite,   and  regards  as  the  most  acid  member  of  his   monzonite 
family. 


QUARTZ-DIORITES.  63 

points  in  common  with  the  Monzoni  syenites.  The  dominant 
felspar  is  a  striated  plagioclase,  often  shewing  zonary  banding 
and  with  a  strong  tendency  to  idiomorphic  outlines ;  but  there 
is  frequently  clear  orthoclase  in  addition,  in  irregular  crystal 
plates  moulded  on  or  enclosing  the  triclinic  felspar.  Biotite 
is  the  most  constant  coloured  element,  but  hornblende  is 
also  abundant.  The  mutual  relations  of  the  two  are  variable, 
and  both  may  enclose  the  plagioclase.  Interstitial  quartz  is 
abundant;  patches  of  magnetite  are  often  prominent;  and 
zircon  in  little  well-built  prisms  is  of  general  occurrence. 

Many  of  the  Scottish  '  granites '  of  Upper  Palaeozoic  age 
are  better  classed  as  quartz-diorites,  and  shew  well  the  inter- 
stitial quartz,  the  zoned  plagioclase  crystals,  and  other  charac- 
teristic features.  A  good  quartz-mica-diorite  comes  from  the 
lower  part  of  Beinn  Nevis.  Other  quartz-diorites  occur  about 
Garabal  Hill,  near  the  head  of  Loch  Lomond1,  and  shew 
gradations  on  the  one  hand  into  granites,  on  the  other  into 
quartzless  diorites  (including  mica-diorite  and  augite-diorite). 
Similar  gradations  are  exhibited  by  the  Beinn  Cruachan  rock, 
which  is  in  the  main  of  the  Tonale  type2.  Of  the  three  main 
masses  of  the  *  Galloway  granites '  one,  that  of  Criffel,  has  as 
its  prevalent  variety  a  quartz-diorite  of  the  same  type. 

In  Wicklow,  to  the  east  of  Rathdrum,  occur  quartz-diorites 
and'quartz-mica-diorites  which  in  some  particulars  approximate 
to  the  granites,  subordinate  orthoclase  accompanying  the 
dominant  triclinic  felspar.  The  other  minerals  are  pale  green 
hornblende,  ragged  flakes  of  biotite,  abundant  quartz,  apatite, 
and  sometimes  a  little  colourless  augite3.  The  augite-diorites, 
a  common  type  in  Wicklow,  sometimes  contain  interstitial 
quartz. 

In  the  United  States,  as  in  Britain,  numerous  rocks 
belonging  here  have  been  designated  granite,  or  sometimes 
granite-diorite.  A  type  with  subordinate  potash-felspar, 
largely  developed  in  the  Sierra  Nevada  of  California,  has 


1  Dakyns  and  Teall,  Q.  J.  G.  S.  (1892)  xlviii,  104-120. 

2  Kynaston,  Ann.  Rep.  Geol.  Sur.  for  1896,  24. 

3  Hatch,  G.  M.  1889,  262,  263  ;  see  also  Watts,  Guide,  34. 


64  MICA-DIORITES. 

been  styled  'granodiorite,'  and  is  regarded  by  Lindgren1  as 
intermediate  between  true  quartz-diorite  and  quartz-monzonite. 
Of  similar  nature  is  the  rock  of  Butte  City,  Mont.,  the  chief 
constituents  of  which  are  acid  labradorite,  orthoclase,  quartz, 
and  green  hornblende,  with  subordinate  biotite2. 

As  a  typical  quartz-diorite  may  be  cited  that  described  by 
Iddings^from  Electric  Peak  in  the  Yellowstone  Park.  Here 
the  dominant  felspar  ranges  from  oligoclase  to  labradorite,  and 
there  is  sometimes  orthoclase  in  addition ;  the  quartz  is  in 
allotriomorphic  grains ;  and  the  other  constituents  are  biotite, 
hornblende,  augite,  hypersthene,  and  magnetite.  Parallel 
intergrowths  are  frequent  among  the  ferro-magnesian  minerals, 
hypersthene  being  bordered  by  augite  and  the  pyroxenes  by 
biotite  and  hornblende4.  A  porphyritic  quartz-mica-diorite 
was  described  by  G.  H.  Williams 5  among  the  varied  dioritic 
rocks  of  the  Cortlandt  district.  The  large  felspar  crystals  are 
strongly  zoned,  but  only  occasionally  lamellated. 

A  mica-diorite,  without  quartz,  is  riot  a  common  type.  It 
is  found  as  a  local  modification  of  biotite-granite  between 
Carrick  Mt.  and  Arklow,  in  Wicklow.  Mr  Teall6  describes  a 
good  example  from  Pen  Voose  in  the  Lizard  district,  Cornwall. 
This  consists  essentially  of  felspar  and  a  reddish  brown  mica 
with  only  quite  subordinate  green  hornblende  and  accessory 
sphene.  From  Allt-a-Mhullin,  south  of  Lochinver,  Sutherland, 
the  same  author  notes  a  mica-diorite  with  interstitial  felspar. 
Among  the  Cortlandt  rocks,  on  the  Hudson  River,  a  pure 
mica-diorite  occurs,  beside  various  mica-hornblende-diorites. 
It  is  a  rather  coarse-grained  aggregate  of  plagioclase  (oligo- 
clase-andesine)  and  very  deeply  coloured  biotite,  with  accessory 
epidote,  magnetite,  abundant  apatite,  and  sometimes  a  little 


1  A.  J.  S.  (1897)  iii,  308-312 ;  see  also  Turner,  17th  Ann.  Rep.  U.  S. 
Geol.  Sur.  (1896)  636,  637,  pi.  XLII,  A. 

2  Weed,  Journ.  Geol.  (1899)  vii,  740-744. 

3  See  12th  Ann.  Rep.  U.  S.  Geol.  Sur.  (1892)  595-609 ;  also  in  Diller, 
243,  244. 

4  Ibid.  pi.  L. 

5  A.  J.  S.  (1888)  xxxv,  446. 

6  PI.  XXXII,  fig.  1  ;    XLVII,  fig.    3. 


HORNBLENDE-DIORITES. 


65 


quartz1.     Mica-diorite  has  been  noted  near  the   Comstock 
Lode,  Nevada. 

Of  simple  hornblende-diorite,  without  quartz,  good  examples, 
of  Palaeozoic  age,  are  found  in  Warwickshire  and  other  parts 
of  the  Midlands.  In  the  rock  of  Atherstone,  Hartshill,  the 
brown  hornblende  is  in  part  idiomorphic  towards  the  turbid 
felspar,  but  part  of  it,  on  the  other  hand,  is  derived  from  a 
colourless  augite,  and  a  kernel  of  the  latter  mineral  sometimes 
remains  unchanged.  Grains  of  magnetite  are  present,  and 
abundant  prisms  of  apatite  (fig.  14).  Mr  Allport2  noted  also 


i 

M  limA 


FIG.  14.     DIORITE,  ATHERSTONE,  WARWICKSHIRE;    x20. 

The  figure  shews  idiomorphic  hornblende  (/«),  turbid  felspar  (/), 
magnetite  (m),  and  rather  abundant  prisms  of  apatite  (op).  Cross- 
sections  of  the  last  shew  the  hexagonal  shape,  and  longitudinal  sections 
shew  the  cross-fracture  [1608]. 

olivine  pseudomorphed  by   carbonates,   etc.     Rather  coarse- 
grained  diorites   are   met  with  in   the   curious   complex   of 

1  G.  H.  Williams,  A .  J.  S.  (1888)  xxxv,  443-445 ;  Kemp,  ibid,  xxxvi, 
247-254. 

-  Q.  J.  G.  S.  (1879)  xxxv,  637-641.  Some  of  the  rocks  included  as 
diorites  by  this  author  would  now  be  classed  with  the  camptonites :  see 
below,  Chap.  X. ;  compare  Watts,  Pr.  GeoL  Ass.  (1893)  xv,  394-396. 

H.  p.  5 


66  HORNBLENDE-DIORITES. 

igneous  rocks  in  the  Malvern  district.  A  specimen  taken 
near  the  New  Reservoir  consists  essentially  of  idiomorphic 
greenish-brown  hornblende  and  labradorite  felspar.  The 
latter  shews  albite-  and  pericline-lamellation,  and  its  decom- 
position has  given  rise  to  zeolites  and  paragonite  mica.  In 
the  well-known  diorite  of  Brazil  Wood1  in  Charnwood  Forest, 
Leicestershire,  the  hornblende  tends  to  embrace  the  felspar, 
and  this  departure  from  the  granitic  type  of  structure  is 
observable  in  some  other  diorites  from  the  Midland  counties. 

Various  diorites  occur  in  the  interior  of  Anglesey.  One 
between  Gwindu  and  Llanfaelog  is  a  coarse-textured  rock  con- 
sisting of  greenish-brown  hornblende  and  turbid  felspar  with 
magnetite  and  apatite.  The  minor  intrusions  near  Llanerchy- 
medd2  are  of  a  rather  different  type.  Brown  hornblende 
occurs  in  well-formed  crystals  and  also  in  shapeless  plates, 
which  can  sometimes  be  seen  forming  at  the  expense  of  a 
colourless  augite.  There  is  also  hornblende  of  later  growth 
than  the  crystals  mentioned  but  not  derived  from  augite. 
It  occurs  as  a  crystalline  outgrowth  of  the  original  brown 
crystals.  Part  of  it  has  grown  upon  the  clinopinacoid  faces 
and  itself  shews  crystal  boundaries;  this  is  green.  Part  has 
grown  chiefly  on  the  terminations  of  the  original  crystals  and 
filled-up  interstices  :  this  is  pale  or  colourless  (fig.  13).  Some 
of  these  rocks  contain  olivine,  or  rather  its  alteration-products, 
and  but  little  felspar,  affording  a  transition  from  diorite  to 
hornblende-picrite3.  Other  oli  vine-bearing  diorites  occur  near 
Clynog-fawr  in  Caernarvonshire 4.  Here  the  hornblende  forms 
ophitic  plates  and  is  probably  in  part  derived  from  augite. 
The  same  remark  applies  to  certain  rocks  at  Penarfynydd 5  in 
the  Lleyn  peninsula,  where  both  ophitic  and  idiomorphic 
augite  may  be  seen  partly  converted  into  brown  hornblende. 
Olivine  seems  to  have  been  rare  in  these  latter  rocks,  but  they 
are  closely  associated  with  a  hornblende-picrite  rich  in  that 

1  Hill  and  Bonney,  Q.  «7V  G.  S.  (1878)  xxxiv,  224. 

2  G.  M.  1887,  546-552.     Other  types  of  dioritic  rocks  from  Central 
Anglesey    are    described    by    Mr    Blake,    Rep.   Brit.   Assoc.    for    1888, 
403-406. 

3  Bonney,  Q.  J.  G.  S.  (1881)  xxxvii,  137-139 ;  (1883)  xxxix,  254-256. 

4  Bala  Vole.  Ser.  Caern.  102-106, 
6  Ibid.  92-97. 


HORNBLENDE-D10RITES.  67 

mineral.  Some  thoroughly  basic  dioritic  rocks,  very  like  those 
of  Anglesey,  occur  in  the  Lake  District,  e.g.  at  Little  Knott1, 
White  Hause,  and  Great  Cockup2  in  the  Skiddaw  district. 
The  rock  at  the  first-named  locality  shews  beautifully  the  pale 
fringes  of  hornblende  which  form  a  crystalline  outgrowth  of 
the  original  idiomorphic  crystals.  These  fringes  are  clearly 
secondary,  and  occupy  the  place  of  destroyed  felspar,  etc. 
Some  olivine  has  been  present  in  some  specimens.  These 
Welsh  and  Cumbrian  dioritic  rocks  occur  usually  in  small 
laccolitic  intrusions,  probably  of  Ordovician  age. 

In  the  Isle  of  Man  several  small  masses  of  diorite  are 
found  on  Langness.  The  hornblende,  of  a  greenish-brown 
tint,  is  perfectly  idiomorphic,  but  often  shews  secondary  out- 
growtbs.  The  felspars  are  much  decomposed.  Abundant 
zoisite,  epidote,  calcite,  etc.,  have  been  produced,  and  the 
quartz  which  is  always  found  is  probably  all  secondary. 
Apatite  is  plentiful,  but  a  little  pyrites  is  usually  the  only 
iron-ore  present. 

The  diorites  of  the  Scottish  Highlands  are  not  yet  de- 
scribed in  any  detail.  Those  of  the  Garabal  Hill  district 
include  mica-diorite  and  augite-diorite.  The  pale  green  augite 
is  usually  in  allotriomorphic  grains  irregularly  bordered  by 
green  hornblende.  Diorites,  with  other  hornblendic  rocks, 
occur  near  Inchnadamff  in  Sutherland3.  Here  the  horn- 
blende is  in  unusually  perfect  crystals. 

In  America  the  Cortlandt  rocks  include  diorites  consisting 
of  brown  hornblende,  andesine,  apatite,  and  magnetite,  some- 
times with  accessory  hypersthene,  and  by  failure  of  the  felspar 
these  rocks  graduate  into  hornblende -rocks.  There  are  also 
diorites  with  green  hornblende 4.  From  Alabama5  are  described 
both  basic  diorites  and  others  of  more  acid  nature,  which  con- 
tain a  little  quartz  and  orthoclase.  The  diorites  described 

1  Bonney,  Q.  J.  G.  S.  (1885)  xli,  511-513,  pi.  xvi,  fig.  2. 

2  Postlethwaite,  Q.  J.  G.  S.  (1892)  xlviii,  510. 

3  Teall,  G.  M.  1886,  346-353. 

4  G.  H.  Williams,  A.  J.  S.  (1888)  xxxv,  441,  442. 

5  Clements,  Bull.  No.  5  Geol.  Sur.  Ala.  (1896)  152-165;  Brooks,  ibid. 
189,  190. 

5—2 


68 


AUGITE-DIORITES. 


by  Zirkel1  from  Nevada  are  chiefly  of  the  more  acid  kind, 
sometimes  carrying  quartz  or,  again,  passing  into  mica-diorite 
(Pah-Ute  range).  The  diorites  of  the  great  laccolitic  masses 
in  Colorado,  Utah,  and  Arizona,  of  which  Cross2  has  given  a 
full  account,  are  also  of  relatively  acid  varieties,  with  quartz, 
and  tend  to  take  on  a  porphyritic  structure,  graduating  into 
quartz-porphyrites. 

A  number  of  dioritic  rocks  may  be  studied  in  the  Channel 
Islands.  A  very  fresh  rock  from  the  quarries  of  Delancy  Hill, 
Guernsey,  is  an  augite-diorite,  with  colourless  augite  as  well  as 
brown  original  hornblende.  The  latter  mineral  is  moulded  on 
the  felspar-prisms,  and  often  borders  the  augite  with  the  usual 
crystallographic  relation  (fig.  15).  A  specimen  from  Rope- 
walk  Quarry  is  also  an  augite-diorite  with  diabasic  characters. 
The  colourless  augite  is  partly  in  rounded  grains  enclosed  by 


FIG.  15.     AUGITE-DIORITE,  DELANCY  HILL,  GUERNSEY  ;    x  20. 

The  augite  shews  either  sharp  octagonal  cross-sections  (a)  or  more 
rounded  contours  (a').  Hornblende  (h),  magnetite  (m),  and  clear  plagio- 
clase  felspar  (/)  are  the  other  constituents.  Much  of  the  hornblende 
occurs  in  marginal  intergrowth  with  the  augite,  interposed  between  the 
latter  mineral  and  the  magnetite  [431]. 

1  Micro.  Petrogr.  Fortieth  Parallel  (1876),  85-93. 

2  Laccolitic  Mountain  Groups,  Uth  Ann.  Rep.  U.  S.  Geol.  Sur.  (1895). 


ESSEXITES.  60 

the  felspar,  partly  in  shapeless  plates,  and  the  brown  horn- 
blende, apparently  an  original  mineral,  is  clearly  of  posterior 
consolidation  to  the  felspar.  Magnetite  is  plentiful,  and  there 
are  some  large  crystals  of  a  rhombic  pyroxene  replaced  by 
bastite.  An  augite-diorite  from  Fort  Touraille,  in  Alderney, 

§ives  evidence  of  the  conversion  of  augite  into  hornblende, 
ome  deep  brown  biotite  is  also  present,  and  a  little  interstitial 
quartz  is  the  last  product  of  consolidation. 

The  essexite  of  Sears1,  occurring  in  association  with  the 
nepheline-syenite  of  Salem,  Mass.,  may  be  regarded  as  a 
peculiar  olivine-augite-diorite  allied  to  the  theralites.  The 
pale  green  augite  is  bordered  by  brownish  hornblende,  and 
brown  biotite  is  intimately  associated  with  them.  The  rounded 
grains  of  olivine  are  often  pseudomorphed  by  biotite-aggregates, 
green  hornblende,  and  granular  augite.  The  iron-ore  is  tita- 
niferous,  and  gives  rise  to  secondary  sphene.  Apatite  is 
abundant  in  irregular  grains  as  well  as  in  slender  prisms. 
The  felspar,  in  idiomorphic  crystals,  is  chiefly  an  acid  labra- 
dorite,  but  a  subordinate  amount  of  alkali -felspar  is  also 
present,  and  perhaps  some  nepheline.  A  similar  rock  is 
found  at  Mount  Royal  and  other  places  near  Montreal2. 
Here  the  augite  is  of  a  reddish-violet  colour,  probably  tita- 
niferous.  The  rock  passes  into  a  theralite  carrying  both 
nepheline  and  sodalite.  An  essexite  has  been  described  from 
the  Rosita  Hills,  Colorado3.  Brogger's  rock  from  Gran,  in  the 
Christiania  basin,  is  similar.  Here  hornblende  is  wanting, 
the  dominant  coloured  silicate  being  a  violet  titaniferous 
augite4. 

1  Bull.  Essex  Inst.  (1891)  xxiii :  see  also  Washington,  Journ.  Geol. 
(1899)  vii,  52-64. 

2  Dresser,  Amer.  Geol.  (1901)  xxviii,  205,  206  (Shefford  Mt.). 

3  Cross,  Proc.  Colo.  Set.  Soc.  (1887)  246,  247. 

4  Q.  J.  G.  S.  (1894)  1,  18. 


CHAPTER  V. 

GABBROS  AND  NORITES. 

THE  gabbros  and  their  allies  are  holocrystalline  rocks, 
typically  of  plutonic  habit,  in  which  the  essential  constituents 
are  a  lime-soda-felspar  and  a  pyroxene.  Of  intermediate  to 
thoroughly  basic  character,  they  correspond  partly  with  the 
diorites ;  but  the  more  acid,  and  especially  the  quartz-bear- 
ing types,  are  less  represented  in  the  pyroxenic  than  in  the 
hornblendic  series.  According  to  the  dominant  pyroxene,  we 
recognize  gabbro1  proper  (euphotide  of  Haiiy)  with  diallage  or 
augite,  and  norite  (also  called  hypersthenite2)  with  a  rhombic 
pyroxene.  A  few  of  the  more  acid  rocks  contain  free  silica 
(quartz-gabbro  arid  quartz- norite].  In  most  of  the  more 
basic  varieties  olivine  becomes  a  characteristic  mineral  (plivine- 
gabbro  and  olimne-nwite).  The  majority  of  the  rocks  in  this 
family  contain  more  or  less  olivine,  and  the  mineral  may  be 
present  or  absent  in  different  specimens  of  the  same  mass. 

The  gabbros  and  norites,  indeed,  shew  considerable  varia- 
tions in  mineralogical  constitution  in  parts  of  one  mass,  and 
most  of  the  special  types  are  probably  to  be  regarded  as  merely 
local  modifications.  Thus,  by  the  failure  of  one  or  other  of 
the  chief  constituents  of  a  gabbro  we  may  have  an  almost 
pure  felspar-rock  (labrador-rock,  anorthosite)  or  pyroxene-rock 

1  Gabbros  in  which  the  felspathic  element  is  anorthite  have  sometimes 
been  termed  eucrite. 

2  In  many  of  the  '  hypersthenites '  of  the  older  writers  the  supposed 
hypersthene  is  only  a  highly  schillerized  diallage. 


FELSPAR   OF   GABBROS.  71 

(diallage-rock,  etc.,  'pyroxenite'  of  Williams1).  By  the  dis- 
appearance of  pyroxene  in  an  olivine-gabbro  we  have  the 
so-called  troctolite  (Ger.  Forellenstein),  composed  essentially 
of  felspar  and  olivine  :  with  abundant  olivine  and  diminishing 
felspar  we  have  transitions  to  the  succeeding  family  of 
peridotites. 

The  name  hornblende- gabbro  has  been  used  for  rocks  of 
this  family  which  contain  hornblende  in  addition  to  pyroxene, 
or  in  which  original  pyroxene  is  more  or  less  completely 
replaced  by  hornblende2.  When  the  conversion  is  complete 
we  have  no  decisive  criterion  for  verifying  the  derivative 
nature  of  the  hornblende,  and,  as  already  remarked,  the  dis- 
tinction between  diorite  and  gabbro  is  a  somewhat  artificial 
one3. 

A  historical  account  of  the  classification  of  the  gabbros 
and  allied  rocks  has  been  given  by  Bayley4. 

Constituent  minerals.  The  felspar  of  the  gabbros 
and  norites  ranges  in  different  examples  usually  from  labra- 
dorite  to  anortliite.  It  builds  large  irregularly-shaped  plates 
with,  as  a  rule,  rather  broad  lamella}5  (albite  twinning)  often 
crossed  by  fine  pericline-striation.  The  lamella}  not  infre- 
quently have  something  of  a  wedge-shape".  A  crystal  with 
broad  albite  lamella),  if  cut  nearly  parallel  to  the  brachy- 
pinacoid,  may  appear  untwinued.  It  is  not  safe  to  assume 
that  the  most  constant  twin-lamellation  necessarily  corresponds 
with  the  albite  law :  the  felspar  of  some  rocks  of  this  family 
has  pericline  twinning  alone  or  predominant. 

Zonary  structure  is  typically  not  found.  Besides  fluid- 
pores  and  inclusions  of  earlier  products  of  crystallization,  the 
felspars  often  shew  more  or  less  marked  schiller-structure7 

1  Amer.  Geol.  (1890)  vi,  40-49.     Williams  regarded  the  pyroxenites  as 
a  group  coordinate  with  the  peridotites.     The  name  is  ill-chosen,  having 
been  employed  in  two  or  three  other  quite  different  senses. 

2  R.  D.  Irving,  Copper-bearing  Rocks,  L.  Superior,  56-58,  pi.  vn. 

3  Prof.  Cole  restricts  the  name  gabbro  to  the  olivine-bearing  (corre- 
sponding  roughly  to   the   basic)  division,  and   styles  the  intermediate 
felspar-augite-rocks  '  augite-diorite. ' 

*  Journ.  Geol.  (1893)  i,  435-456. 

5  Cohen  (3),  pi.  xxv,  fig.  3.  «  Ibid.  pi.  xxvi,  fig.  2. 

i  Ibid.  pi.  v,  fig.  2. 


72 


AUGITE    OF   GABBROS. 


(tig.  1,  g).  The  modes  of  alteration  of  the  felspars  are 
various  :  Rosenbusch  notes  the  curious  fact  that  calcite  is 
seldom  formed.  The  '  saussurite '  change  seems  to  belong 
often  to  dynamic  metamorphism  rather  than  to  weathering 
(see  below,  Chap.  XXL).  Any  plagioclase  more  acid  than 
labradorite  is  exceptional,  and  so  is  the  occurrence  of  ortkoclase 
(e.g.  Carrock  Fell  and  Lake  Superior  region1). 

The  augite  of  the  gabbros  builds  irregular  crystal-plates 
and  wedges  of  very  pale  green  or  light  brown  colour.  Besides 
the  usual  prismatic  cleavage,  an  orthopinacoidal  cleavage  and 
diallage-structuYe  are  very  common2.  Instead  of  this,  there 
is  sometimes  a  very  minute  striation  parallel  to  the  basal 
plane.  The  common  twin,  parallel  to  the  orthopinacoid,  is 
often  associated  with  this  (fig.  16,  A).  Decomposition  gives 


li. 


FIG.  16.  PYROXENES  IN  THE  GABBKO  OF  CAKKOCK  FELL, 

CUMBERLAND  ;  x  20. 

The  dominant  variety  is  an  augite  with  basal  striation.  A  shews  this 
structure  combined  with  twinning  on  the  orthopinacoid  to  give  the 
'herring-bone'  structure.  The  mineral  is  partly  converted  to  green 
hornblende  [1870J.  B  shews  a  parallel  intergrowth  of  the  augite  with 
enstatite,  the  latter  mineral  forming  the  core  and  the  former  the  outer 
shell,  but  with  detached  portions  of  augite  enclosed  in  the  enstatite  in 
micrographic  fashion  [2279]. 

1  B.  D.  Irving,  Copper-bearing  Rocks  of  L.  Superior,  50-55,  pis.  v,  vi. 

2  Cohen  (3),  pi.  XLII,  fig.  3. 


HYPERSTHENE   OF   GABBROS   AND    NORITES.  73 

a  scaly  or  fibrous  aggregate  of  chlorite  and  serpentine  with 
other  products.  Another  common  alteration  is  the  conversion 
to  hornblende1,  which  may  be  light  green  and  fibrous  (uralite) 
or  deep  brown  and  compact. 

The  rhombic  pyroxenes,  bronzite  and  hypersthene,  occur  as 
accessory  minerals  in  rather  rounded  but  allotriomorphic 
crystals,  while  in  the  norites  they  often  shew  but  little  crystal- 
outline.  A  schiller-structure2  is  common  in  many  norites  and 
gabbros  (fig.  17).  The  most  usual  alteration  is  into  distinct 
pseudomorphs  of  the  serpentinous  mineral  bastite.  This  is 
pale  green  or  yellowish  with  slight  pleochroism  and  low 
polarization-tints.  The  pseudomorph  is  built  of  little  fibres 
arranged  longitudinally,  and  is  traversed  by  irregular  cracks 


FIG.  17.     NORITE  (HYPERSTHENITE),  COAST  OF  LABRADOR  ;    x  20. 

Consisting  of  bypersthene  (hy),  felspar  (an),  and  apatite  (a^).  Schiller- 
inclusions  are  strongly  developed  in  the  hypersthene  and  to  a  less  extent 
in  the  felspar  [G  444]. 

which  the  fibres  do  not  cross  (see  fig.  23).     The  individual 
fibres   give   straight   extinction,   but,   as   there    is    a   slight 

1  See  G.  H.  Williams,  A.J.  S.  (1884)  xxviii,  261-264  ;  Bull.  No.  28 
U.  S,  Geol.  Sur.  (1886). 

2  Cohen  (3),  pi.  v,  fig.  3. 


74  OLIVINE   OF   GABBROS. 

departure  from  perfect  parallelism  in  their  arrangement,  a 
very  characteristic  appearance  is  offered.  The  rhombic  pyro- 
xenes also  shew  uralitization. 

In  the  rocks  here  included  original  hornblende  is  found 
only  as  an  occasional  accessory :  a  deep  brown  variety  occurs 
in  some  norites.  Brown  biotite  may  also  occur  as  a  minor 
accessory  (e.g.  Carrock  Fell ;  St  David's  Head),  and  it  may 
be  intergrown  with  augite  (Stanner  Rock,  near  New  Radnor1). 

When  olivine  is  present,  it  builds  imperfect  crystals  or 
rounded  grains,  colourless  in  slices.  Where  it  adjoins  felspar, 
it  is  often  bordered  by  a  rirn  of  hypersthene.  The  olivine 
sometimes  has  schiller-inclusions. 

The  characteristic  mode  of  alteration  of  olivine  is  '  serpen- 
tinization. '  This  process  begins  round  the  margin  of  the 
crystal-grain  and  along  the  usually  irregular  network  of 
cracks  which  traverses  it.  Along  these,  as  a  first  stage, 
strings  of  granular  magnetite  separate  out.  The  immediate 
walls  of  the  cracks  are  converted  into  pale  greenish  or  yellowish 
fibrous  serpentine,  the  fibres  set  perpendicularly  to  the  crack, 
and  giving  straight  extinction  and  low  polarization-tints.  At 
this  stage  the  meshes  of  the  network  are  occupied  by  unaltered 
remnants  of  olivine.  These  may  be  subsequently  altered  to 
serpentine,  which  is  of  a  different  character  from  that  first 
formed,  being  often  sensibly  isotropic2.  As  a  last  stage,  some 
of  the  magnetite  may  be  reabsorbed,  giving  a  deeper  colour  to 
the  serpentine  pseudomorph.  The  change  from  olivine  to 
serpentine  involves  an  increase  of  volume,  which  gives  rise 
to  numerous  radiating  cracks  traversing  adjacent  minerals3. 
These  cracks  are  injected  with  serpentine,  usually  isotropic 
(fig.  18). 

Where  original  quartz  occurs  in  gabbros,  etc.,  it  has  the 
same  properties  as  that  in  granites.  Usually  it  forms  part  of 
a  micrographic  intergrowth. 

1  Cole,  G.  M.  1886,  p.  221,  fig.  3. 

2  This  effect  is  possibly  due  to  the  overlapping  of  a  crowd  of  minute 
fibres  or  scales  without  any  definite  orientation.     For  successive  stages 
of  serpentinization  of  olivine  see  Cohen  (3),  pi.  LIX. 

3  Cohen  (3),  pi.  LXXVII,  fig.  4. 


STRUCTURE    OF   GABBROS. 


75 


Original  iron-ores  occur  only  sparingly  in  some  rocks  of 
the  gabbro  family,  but  sometimes  become  abundant.  They 
are  ilmenite  (with  leucoxene  as  a  decomposition-product)  and 
magnetite.  In  some  cases  brown  grains  of  picotite  are  found. 


FIG.  18.     LABKADORITE-OLIVINE-KOCK  (TKOCTOLITE),  COVERAGE  COVE, 
CORNWALL  ;    x  20. 

The  olivine  is  almost  wholly  converted  into  serpentine  (a  few  clear 
granules  remaining),  and  the  consequent  expansion  has  caused  radiating 
fissures  through  the  surrounding  felspar  [1116]. 

The  apatite  builds  the  usual  hexagonal  prisms  or  sometimes 
short  rounded  grains  (fig.  17).  In  other  accessories  the 
rocks  are  usually  very  poor,  zircon  and  original  sphene  being 
absent. 

Structure.  In  texture  the  rocks  of  this  family  vary 
from  medium  to  coarse  grain.  In  some  the  individual  crystals 
of  felspar  and  pyroxene  attain  a  large  size,  and  they  are  then, 
as  a  rule,  strongly  affected  by  schiller-structures.  Porphyritic 
structure  is  very  rarely  met  with  in  the  gabbros  and  norites. 

The  order  of  crystallization  is  in  general  less  decisively 
marked  in  basic  than  in  acid  rocks.  This  seems  to  be  due  to 
the  periods  of  crystallization  of  the  several  minerals  having  in 


76  STRUCTURE   OF   GABBROS. 

great  measure  overlapped.  The  relative  idiomorphism  of  the 
crystals  only  indicates  the  order  in  which  they  ceased  to  form, 
not  that  in  which  they  began.  It  is  only  with  this  under- 
standing that  the  rocks  of  the  gabbro  family  can  be  said  to 
follow  the  normal  law.  Apatite,  iron-ores,  and  olivine,  when 
present,  are  the  earliest  minerals,  and  are  clearly  idiomorphic, 
while  in  the  special  types  containing  orthoclase  and  quartz 
these  minerals  have  always  crystallized  last.  But  as  regards 
the  two  main  constituents,  augite  and  plagioclase,  the  mutual 
relations  are  not  always  the  same.  In  many  gabbros  the 
felspar  is  more  or  less  distinctly  embraced  by  the  augite  or 
diallage,  but  if  this  character  becomes  marked  there  are  always 
other  features  which  indicate  a  transition  to  the  diabase  type. 
The  more  typical  gabbros  are  often  thoroughly  hypidiomorphic ; 
or  the  augitic  constituent,  especially  if  very  abundant,  may  be 
embraced  by  the  felspar.  When  a  rhombic  pyroxene  enters, 
it  is  idiomorphic  towards  the  monoclinic,  and  usually  towards 
the  felspar  also. 


FlG.   19.       OHVINE-NORITE    WITH    CORONA-STRUCTURE,    SEILAND, 

NEAR  HAMMERFEST  ;    x  15. 

A  much-fissured  crystal  of  olivine  (ol)  is  surrounded  by  a  continuous 
ring  of  hypersthene  (hy)  interposed  between  it  and  the  anorthite  felspar  (/). 
There  is  a  little  brown  hornblende  (ho)  and  some  brown  biotite  (b)  clinging 
about  the  iron-ore  grains  [418]. 


CORONA  AND  CELYPHITE  STRUCTURES.       77 

In  many  plutonic  rocks  there  is  an  evident  tendency  for 
the  earlier  formed  minerals  to  serve  as  nuclei  round  which  the 
later  ones  have  crystallized.  This  tendency  is  most  marked  in 
basic  and  ultrabasic  rocks.  Thus  in  gabbros  and  norites  the 
pyroxenes  often  form  a  more  or  less  continuous  ring  or 
'corona'  round  olivine  or  iron-ores  (fig.  19).  This  is  the 
'celyphytic  structure'  of  Rosenbusch.  Bayley1,  while  noting 
this  feature,  further  describes  fibrous  intergrowths  of  felspar 
and  augite  surrounding  olivine  or  magnetite.  These  seem 
to  be  original,  but  in  other  cases  there  is  reason  to  believe 
that  a  mineral  bordering  another  one  is  of  secondary  origin. 
Good  examples  are  figured  and  described  by  G.  H.  Williams2 
in  the  hypersthene-gabbros  of  the  Baltimore  district.  Here 
both  hypersthene  and  diallage  are  surrounded  by  a  double 
*  reaction-rim '  of  hornblende,  interposed  between  the  pyroxene 
and  the  felspar  and  due  to  a  reaction  between  them.  The 
inner  zone  of  the  rim  is  of  fibrous,  the  outer  of  compact  horn- 
blende. They  are  apparently  the  beginning  of  a  process  by 
which  the  pyroxenes  are  eventually  wholly  transformed  into 
green  hornblende,  and  the  author  named  considers  that  they 
do  not  necessarily  imply  dynamic  metamorphism.  In  many 
cases  there  seems  to  be  no  decisive  evidence  as  to  the  primary 
or  secondary  origin  of  the  interposed  minerals. 

Gabbros  with  granulitic  structure  occur  in  many  districts, 
sometimes  in  intimate  association  with  those  of  more  normal 
type.  Most  of  the  rocks  styled  pyroxene-granulites  probably 
fall  under  this  head,  but  we  defer  noticing  these  until  a  later 
chapter  (Chap.  XXII.). 

Leading  types.  We  begin  with  the  rather  exceptional 
rocks  in  which  free  silica  has  been  developed  as  an  original 
constituent.  A  good  example  of  a  quartz-gabbro  is  that  of 
Carrock  Fell,  in  Cumberland3.  It  consists  mainly  of  a  some- 
what basic  labradorite  and  an  augite  with  basal  striation. 

1  Amer.  Journ.  Sci.  (1892)  xliii,  515-518 :  Journ.  of  Geol.  (1893)  i, 
702-710. 

2  Bull.  No.  28,  U.  S.  Geol.  Sur.  (1886),  with  plates. 

3  Q.  J.  G.  S.  (1894)  1,  316-318,  pi.  xvn  :  (1895)  li,  125.     The  rock  has 
been  termed  hypersthenite,  hut  the  rhombic  pyroxene  is  always  subordi- 
nate to  the  mouoclinic  and  sometimes  wanting. 


78  QUARTZ-GABBROS. 

Imperfect  prisms  of  enstatite  also  occur,  and  there  is  often 
a  parallel  intergrowth  of  the  two  pyroxenes  (fig.  16,  B).  The 
augite  is  often  converted  into  a  greenish  fibrous  hornblende 
and  the  enstatite  into  bastite.  Biotite  is  found  locally.  Mag- 
netite and  ilmenite  occur,  sometimes  in  evident  intergrowths. 
Quartz  is  found  partly  in  interstitial  grains  but  chiefly  in 
micrographic  intergrowth  with  felspar,  some  of  which  is 
orthoclase.  The  rock  varies  much,  the  central  part  of  the 
mass  being  rich  in  quartz,  while  the  margin  is  highly  basic, 
free  from  quartz  and  remarkably  rich  in  iron-ores  and  apatite. 
The  mutual  relations  of  the  felspar  and  augite  vary,  but  on 
the  whole  the  augite  tends  to  envelope  the  felspar.  Specimens 
of  the  gabbro  of  St  David's  Head,  also  intrusive  in  Lower 
Palaeozoic  strata,  are  identical  with  the  rock  just  described, 
except  that  the  highly  basic  modification  is  not  found. 
Biotite  is  rather  more  plentiful,  and  the  quartz  and  micro- 
pegmatite  occur  rather  more  sparingly.  The  rhombic  pyroxene 
is  represented  by  pseudomorphs  of  pleochroic  green  bastite, 
always  abundant. 

Hypersthene-bearing  quartz-gabbros  are  extensively  de- 
veloped near  Wilmington,  Delaware1.  Some  varieties  have 
biotite,  and  by  increase  in  the  proportion  of  this  mineral  pass 
into  the  type  which  Chester  styles  '  gabbro-granite.'  In  other 
varieties  brown  hornblende  becomes  a  conspicuous  mineral, 
but  this  is  probably  formed  at  the  expense  of  the  diallage. 

The  well-known  rocks  of  the  Lizard  district2  in  Cornwall 
are,  for  the  most  part,  simple  gabbros  without  olivine,  although 
that  mineral  occurs  in  some  varieties.  Judging  from  the 
cases  in  which  precise  determinations  have  been  made,  the 
felspar  seems  to  be  labradorite  in  the  less  basic  rocks,  anorth- 
ite  in  the  most  basic.  It  shews  broad  albite-lamellse,  often 
crossed  by  others  following  the  pericline  law.  The  pyroxene 
varies  from  a  pale  green  diopside,  almost  colourless  in  slices, 
to  typical  diallage,  the  diallagic  structure  being  often  seen  to 
affect  only  part  of  a  crystal.  The  enstatite-group  is  wanting 


1  Chester,  Butt.  No.  59,  U.  S.  Geol.  Sur.  (1890). 

2  Teall,  G.  M.  1886,  483-485.     For  description  of  particular  varieties 
see  Bonney,  Q.  J.  G.  S.  (1877)  xxxiii,  884-915,  and  other  papers. 


GABBROS   WITHOUT   OLIVINE.  79 

or  rare.     When   olivine  occurs  it  builds   colourless   grains, 
shewing  various  stages  of  serpentinization. 

The  Lizard  gabbros  exhibit,  however,  numerous  modifica- 
tions which  are  ascribed  to  dynamic  metamorphism,  especially 
the  conversion  of  the  felspar  to  '  saussurite '  and  of  the  augite 
to  amphibole.  The  minutely  granular  mineral-aggregate  known 
as  saussurite  is  opaque  in  any  but  the  thinnest  slices,  and  can 
be  studied  only  under  high  magnifying  powers.  The  change 
may  be  seen  to  begin  in  spots  in  the  felspar  crystals  and 
spread  to  the  whole.  The  pyroxene  passes  over  into  uralitic 
or  actinolitic  or  compact  hornblende  in  different  cases1,  the 
secondary  amphibole  being  pale  green  or  brown  or  colourless, 
or  sometimes  having  a  bright  emerald-green  colour  (smarag- 
dite).  According  as  one  or  both  of  these  changes  have 
affected  the  original  felspar-pyroxene-rock,  we  have  saussurite- 
diallage-gabbro,  felspar-hornblende-gabbro,  or  saussurite-horn- 
blende-gabbro.  At  Karakclews  occurs  a  rock  consisting  of  a 
fine-grained  aggregate  of  augite  (malacolite),  labradorite,  sphene, 
and  an  unknown  substance,  brown  by  transmitted  and  white 
by  reflected  light.  Mr  Teall2  states  that  much  of  the  so-called 
saussurite  of  the  Lizard  is  similar  to  this  rock  in  composition. 
Another  mineral  considered  to  be  of  secondary  origin  is  the 
rhombic  amphibole  anthophyllite3.  This  sometimes  occurs 
in  colourless  and  rather  fibrous  crystals,  forming  a  zone  round 
grains  of  altered  olivine,  and  surrounded  in  turn  by  an  outer 
zone  of  green  actinolite. 

Gabbros  without  olivine  are  met  with  in  Canada,  New 
Hampshire,  and  other  parts  of  America.  Some  from  the 
north-western  part  of  the  Adirondacks,  N.Y.4,  consist  essen- 
tially of  felspar,  in  general  labradorite,  and  augite,  often 
transformed  to  compact  hornblende.  Usually  the  ferro-mag- 
nesian  mineral  predominates,  but  there  are  rapid  transitions 
to  a  highly  felspathic  type.  Other  gabbros  in  the  same 
district  have  accessory  hypersthene.  Gabbros  without,  as 
well  as  others  with,  olivine  are  largely  developed  in  the  Lake 
Superior  region  and  the  neighbouring  parts  of  Minnesota, 

1  Teall,  pi.  xvm,  fig.  2. 

2  M.  M.  (1888)  viii,  118. 
8  Teall,  ibid.  119. 

4  Smyth,  Bull.  Geol.  Soc.  Amer.  (1895)  vi,  263-284. 


80  HORNBLENDE-GABBROS. 

Wisconsin,  etc.1  An  interesting  type  is  the  orthoclase-gabbro 
of  Irving2,  in  which  the  plagioclase  felspar  is  oligoclase  or 
an  allied  variety,  and  some  orthoclase  occurs  in  addition. 
The  augite  may  be  diallagic  and  is  often  uralitized ;  apatite 
is  abundant;  and  the  iron-ore  is  a  highly  titaniferous  mag- 
netite (Duluth  and  Lester  River,  Minn.,  etc.). 

Among  rocks  which  have  been  styled  kornblende-gabbro, 
some  examples  from  Guernsey  (Bellegreve)  exhibit  very 
beautifully  the  conversion  of  colourless  augite  into  brown 
or  greenish-brown  compact  hornblende,  the  process  being 
seen  in  every  stage.  In  some  slides  no  augite  remains,  and, 
without  comparison  with  other  specimens,  the  rock  might 
be  taken  for  a  true  diorite,  but  the  hornblende  is  probably 
all  derivative.  The  ferro-magnesian  silicates  are  often  moulded 
on  the  felspar,  which  is  of  a  basic  variety.  Magnetite  and 
apatite  are  the  only  other  constituents. 

Another  good  illustration  is  furnished  by  the  rocks  styled 
'  gabbro-diorite '  by  Williams  in  the  Baltimore  district3.  These 
have  been  originally  hypersthene-bearing  gabbro,  and  the 
transformation  of  both  pyroxenes  into  green  hornblende,  fibrous 
or  compact,  can  be  traced  step  by  step.  The  process  is  equally 
well  displayed  in  some  of  the  Cortlandt  norites4.  A  good 
hornblende-gabbro,  with  compact  brown  hornblende,  occurs 
near  Bad  River,  Wisconsin5. 

The  Tertiary  igneous  rocks  of  the  Inner  Hebrides  (Skye, 
Rum,  Mull,  etc.)  include  numerous  olivine-gabbros,  and  accord- 
ing to  Prof.  Judd6  most  of  the  gabbros  there  carry  olivine, 
though  that  mineral  may  be  obscured  by  secondary  magnetite. 
The  augite,  as  a  rule,  has  a  striation  parallel  either  to  the 
basal  plane  or  to  the  orthopinacoid,  with  more  or  less  marked 
schillerization  ;  but  the  author  named  has  shewn  how  in 

1  Wadsworth,  Prelim.  Descr.  of  the  Perid.,  Gabbros,  etc.,  of  Minn. 
(1887) ;  R.  D.  Irving,  Copper-bearing  Rocks  of  L.  Superior,  Monog.  No.  5 
U.  S.  Geol.  Sur.  (1884). 

2  E.  D.  Irving,  I.e.  50-56,  pis.  v,  vi. 

3  Bull.  No.  28,  U.  S.  Geol.  Sur.  (1886)  with  plates  :  abstr.  in  G.  M. 
1887,  87,  88. 

4  G.  H.  Williams,  A.J.S.  (1884)  xxviii,  261-264,  with  figures. 

5  K.  D.  Irving,  I.e.  56-58,  pi.  vn,  figs.  1-3. 
e  Q.  J.  G.  S.  (1886)  xlii,  49-89,  pi.  iv. 


OLIVINE-GABBROS.  81 

the  more  deep-seated  portions  of  the  large  rock-masses 
schiller-structures  come  in  in  more  than  one  direction,  and 
affect  the  felspar  and  olivine  as  well  as  the  pyroxene.  A 
rhombic  pyroxene  is  only  locally  present.  The  olivine  is  often 
of  a  variety  rich  in  iron,  and  gives  rise  to  much  magnetite-dust 
as  an  alteration -product.  Original  iron-ores  and  apatite  may 
or  may  not  be  present.  The  felspar  is  usually  a  labradorite, 
and  this,  rather  than  the  pyroxene,  tends  to  assume  crystal 
outlines,  the  structure  of  the  rock  being  often  ophitic,  and  the 
gabbro  graduating  into  diabase. 

Very  similar  to  the  Scottish  Tertiary  gabbros  are  those  of 
the  Carlingford  district  in  Ireland,  probably  of  like  age. 
Prof,  von  Lasaulx1  described  specimens  consisting  of  anorthite, 
diallage,  and  olivine.  These  were  from  Slieve  Foy.  From 
the  neighbouring  hill  of  Barnavarve  Prof.  Sollas2  describes  a 
gabbro  free  from  olivine,  consisting  of  a  basic  felspar  (anorthite 
or  bytownite)  with  rhombic  and  monoclinic  pyroxenes,  which 
shew  rather  remarkable  intergrowths.  Here  is  also  a  variety 
of  the  rock  containing  interstitial  micro-pegmatite,  which  the 
author  named  believes  to  be  due  to  a  later  injection. 

Among  American  olivine-gabbros3  those  described  by 
Irving4  from  the  Lake  Superior  region  tend  to  the  ophitic 
type  of  structure.  The  felspar  is  usually  anorthite  or  some 
other  basic  variety;  the  augite  sometimes,  but  not  always, 
shews  the  diallage  character;  the  iron-ore,  often  in  large 
grains,  is  magnetite  only  slightly  titaniferous ;  and  apatite 
is  rare.  A  rock  from  Pigeon  Point,  Minn.5,  consists  of  fresh 
labradorite,  purplish  pink  titaniferous  augite,  olivine,  tita- 
niferous magnetite,  and  a  little  apatite.  One  modification 
contains  large  porphyritic  crystals  of  the  felspar.  The  large 
gabbro  mass  at  the  base  of  the  Keweenaw  formation  in 
north-eastern  Minnesota6  consists  essentially  of  a  basic 
labradorite,  augite,  an  olivine  rich  in  iron  (hyalosiderite), 

1  Sci.  Proc.  Roy.  Dubl.  Soc.  (1878)  ii,  31-33. 

2  Tr.  Roy.  Ir.  Acad.  (1894)  xxx,  482-487. 

3  For  coloured  plate  of  example  from  New  Hampshire  see  Berwerth, 
Lief.  ii. 

4  Copper-bearing  Rocks  of  L.  Superior  (1884),  with  coloured  plates. 
3  Bayley,  Bull.  No.  109,  U.  S.  Geol.  Sur.  (1893)  32-38,  pi.  v. 

«  Bayley,  Journ.  of  Geol.  (1893)  i,  696-714. 

H.  p.  6 


82  HYPERSTHENE-GABBROS. 

and  a  non-titaniferous  magnetite ;  but  wide  differences  in 
the  relative  proportions  of  these  constituents  give  rise  to 
numerous  varietal  forms. 

As  already  intimated,  many  of  the  rocks  in  this  family 
contain  both  augite  (or  diallage)  and  hypersthene  in  varying 
proportions,  and  no  hard  line  is  to  be  drawn  between  gabbros 
and  norites.  In  Sweden  the  rocks  termed  '  hyperite '  by 
Tornebohm  vary  between  olivine-gabbro  and  aiorite,  olivine 
and  hypersthene  appearing  to  replace  one  another,  so  that  the 
total  of  the  two  remains  about  the  same  in  the  different 
varieties.  The  same  thing  is  seen  in  the  north  of  Norway 
and  elsewhere.  It  is  convenient  to  restrict  the  name  norite 
to  rocks  in  which  the  sole  or  dominant  pyroxene  is  of  a 
rhombic  variety,  those  in  which  the  rhombic  is  only  sub- 
ordinate to  the  monoclinic  pyroxene  being  termed  hypersthene- 
gabbro.  Such  rocks  are  represented  sparingly  among  the 
gabbros  of  Skye  and  Mull.  They  occur  also  in  the  Baltimore 
district ],  in  the  Adirondacks2,  in  Alabama3,  etc.  An  ortho- 
clase-bearing  variety  comes  from  Emigrant  Gap,  Placer  Co., 
Cal.4 

A  good  example  of  qiiartz-norite  is  described  by  Grant5 
from  Mount  Hope,  near  Baltimore.  It  consists  of  bytownite, 
quartz,  and  hypersthene,  with  accessory  magnetite  and  apatite, 
and  has  a  granitoid  structure. 

A  well-known  example  of  norite  comes  from  the  island 
Hittero,  off  the  west  coast  of  Norway.  The  rhombic  pyroxene 
is  a  hypersthene  rich  in  iron ;  but,  as  is  often  the  case,  the 
ferriferous  ingredient  is  concentrated  in  numerous  deep  brown 
schiller-inclusions,  leaving  the  general  mass  of  the  crystal  pale 
and  scarcely  pleochroic.  Some  specimens  have  a  considerable 
amount  of  iron-ore  (probably  titaniferous)  surrounded  by  green 
hornblende. 

1  G.  H.  Williams,  Bull.  No.  28,  V.  S.  Geol.  Sur.  (1886). 

2  C.  H.  Smyth,  \r.,A.  J.  S.  (1894)  xlviii,  54-65;  Bull.  Geol.  Soc.Amer. 
(1895)  vi,  271. 

a  3.  M.  Clements,  Bull,  No.  5,  Geol.  Sur.  Ala.  (1896)  171,  172. 

4  Lindgren,  Bull  No.  148,  U.  S.  Geol.  Sur.  (1897)  212. 

5  Joh.  Hopk.  Univ.  Circ.  (1893)  xii,  48. 


NORITES.  83 

In  Scotland  norites  occur  in  Aberdeen  shire,  Banffshire, 
and  other  districts.  One  from  Towie  "Wood,  near  Ellon, 
consists  essentially  of  labradorite  and  a  rhombic  pyroxene, 
which  is  pale  and  without  schiller-structure  (enstatite) ; 
while  others  from  the  same  neighbourhood  contain  in  addition 
augite,  hornblende,  and  biotite. 

A  well-known  American  example,  with  strongly  schil- 
lerized  hypers thene,  comes  from  the  coast  of  Labrador1 
(fig.  17).  Patches  of  brown  hornblende  and  biotite  are  some- 
times intergrown  with  the  hypersthene.  In  places  it  becomes 
bleached,  with  a  separation  of  granular  magnetite.  The 
other  main  constituent  is  felspar  (usually  typical  labra- 
dorite but  sometimes  a  more  basic  variety),  moulded  on  the 
imperfect  crystals  of  hypersthene.  Stout  prisms  of  apatite 
also  occur,  and  sometimes  patches  of  iron-ore  bordered  by 
brown  mica.  Norites  are  found  also  in  the  Sudbury  district 
of  Canada  and  at  several  localities  in  the  north-eastern  part 
of  the  United  States.  From  the  Adirondacks  Kemp2  describes 
rocks  composed  essentially  of  augite,  hornblende,  hypersthene, 
and  felspar.  Norites  and  allied  types  are  included  among 
the  Cortlandt  rocks  on  the  Hudson  River ;{.  The  norite 
proper  consists  mainly  of  andesine  and  hypersthene,  both 
with  schiller-inclusions.  There  is  accessory  biotite,  and  a 
remarkable  feature  is  the  occurrence  of  large  crystals  of  ortho- 
clase  enclosing  the  other  minerals  in  '  p03cilitic '  fashion.  In 
other  rock-types  from  this  district  the  hypersthene  is  asso- 
ciated with  green  or  brown  hornblende  (hornblende-norite), 
with  biotite  and  magnetite  (mica-norite),  or  with  augite  and 
biotite  (augite-norite). 

In  this  place  may  be  included  the  rocks  to  which  Rosenbusch 
has  given  the  name  theralite,  and  which  may  be  considered  as 
nepheline-gabbros.  The  original  type  is  from  the  Crazy  Mts 
in  Montana4.  Here  olivine  is  only  an  occasional  accessory. 

1  Cohen  (3),  pi.  v,  fig.  3. 

2  IWi  Ann.  Rep.  U.  8.  Geol.  Sur.  (1899)  part  in,  pi.  LX,  A. 

3  G.  H.  Williams,  A.  J.  S.  (1887)  xxxiii,  135-144,  191-194. 

4  Wolff,  Notes  on  the  Petrography  of  the  Crazy  Mts,  etc..  Northern 
Transcontinental  Survey  (1885) ;   and  in  Diller,  197-200.     For  coloured 
plate  see  Berwerth,  Lief.  n. 

6—2 


84  THERALITES:    FELSPAR-ROCKS. 

Hornblende  is  not  present  in  the  typical  rock,  but  the 
idiomorphic  augite,  pale  green  to  almost  colourless  in  slices, 
is  often  surrounded  by  a  narrow  border  of  deep  green 
pleochroic  segirine.  The  felspar  is  partly  unstriated  plagio- 
clase,  partly  perhaps  anorthoclase.  It  forms  with  nepheline 
a  granular  aggregate,  in  which  either  mineral  may  be 
idiomorphic  towards  the  other.  The  remaining  constituents 
are  biotite,  apatite,  and  a  little  iron  ore,  with  sometimes 
sodalite  (Rock  Creek)  or  haiiyne  (Martinsdale).  A  purer 
theralite  is  described  from  Costa  Rica1.  This  consists  of 
augite,  labradorite  and  a  little  orthoclase,  nepheline  and  a 
mineral  of  the  sodalite  group,  biotite,  apatite,  and  magnetite, 
with  secondary  zeolites. 

A  more  remarkable"  rock  is  Pirssori's  missourite*  from  the 
Highwood  Mts,  Montana,  a  leucite-gabbro,  corresponding  with 
the  volcanic  leucite-basalts.  It  is  quite  devoid  of  felspar, 
consisting  of  olivine,  augite,  biotite,  and  leu  cite,  with  some 
apatite  and  iron-ore.  The  structure  is  thoroughly  allotrio- 
morphic. 

The  felspar-rocks  known  in  America  as  anorthosite  must 
be  regarded  as  peculiar  members  of  the  gabbro  family.  Such 
rocks,  of  pre-Cambrian  age,  occupy  extensive  tracts  in  Minne- 
sota3, etc.,  near  Lake  Superior.  The  felspar  which  makes  up 
almost  the  whole  of  these  coarse-textured  aggregates  varies 
from  labradorite  to  anorthite  in  different  localities4.  A  little 
augite,  of  faint  violet-brown  tint  in  sections,  is  the  only 
other  original  mineral,  and  this  occurs  both  in  grains  and  as 
minute  parallel  interpositions  in  the  felspar.  Similar  rocks 
have  been  described  by  Adams  in  the  so-called  Norian  of 
several  districts  in  Canada,  by  Kemp5  in  the  Adirondacks,  etc. 

1  Wolff,  A.  J.  S.  (1896)  i,  271,  272. 

2  A.J.S.  (1896)  ii,  317-323. 

3  K.  D.  Irving,  Copper-bearing  Rocks  of  L.  Superior,  59-61,  pi.  vn, 
fig.  4 ;  Lawson,  Bull.  No.  8,  Geol.  and  Nat.  Hist.  Siir.  Minn.  (1893)  and 
abstr.  in  M.  M.  x,  263.    The  very  coarse-textured  felspar- rock  of  Labrador, 
with  its  beautiful  schiller-structure,  is  in  all  mineralogical  collections. 

4  The  mineralogical  term  'anorthose '  (Delesse),  from  which  anorthosite 
is  named,  is  synonymous  not  with  anorthite  but  with  plagioclase  generally. 

5  Bull.  Geol.  Soc.  Amer.  (1894)  v,  215,  216;  Geology  of  Moriah  and 
Westport,  Bull.  N.Y.  State  Mus.  (181)5)  iii,  337. 


PYROXENE-ROCKS  :    TROCTOLITES.  85 

Iii  our  country  gabbros  pass  only  locally  into  labradorite-rocks 
by  the  failure  of  the  pyroxenic  constituent  (Lenkeilden  Cove 
at  the  Lizard,  Athenree  in  Tyrone1). 

Contrasted  with  these  are  the  pure  pyroxene-rocks  to  which 
Williams  in  America  has  given  the  name  '  pyroxenite.'  The 
Webster  type2  is  described  from  North  Carolina  and  Maryland, 
and  consists  of  a  rhombic  and  a  monoclinic  pyroxene  forming 
an  even-grained  crystalline  aggregate.  It  is  in  fact  a  bronzite- 
diopside-rock.  Another  example,  from  Montana3,  consists  of 
light  green  diallage  and  colourless  enstatite  with  some  brown 
mica  and  only  occasional  felspar.  From  the  same  district 
comes  a  hypersthene-hornblende-rock,  sometimes  rich  in  green 
pleonaste  ;  while  rocks  composed  essentially  of  augite  and 
hornblende  have  been  recorded  from  Alabama4  and  from 
Mariposa  Co.,  Gal.5  Among  other  types  consisting  wholly  of 
ferro-magnesian  silicates  we  may  mention  a  hypersthene-bio- 
tite-rock  from  Hamilton  River  in  Labrador0  and  an  enstatite- 
rock  from  the  Transvaal7.  In  this  country  gabbros  are  only 
locally  known  to  pass  into  augite-  or  diallage-rock  (e.g. 
Lendalfoot  in  Ayrshire8). 

By  the  dwindling  and  disappearance  of  the  pyroxene,  oli- 
vine-gabbros  pass  into  felspar-olivine-rock,  known  as  troctolite 
(Ger.  Forellenstein).  This  consists  essentially  of  a  lime-soda- 
felspar,  typically  labradorite,  with  olivine,  which  may  be  more 
or  less  serpentinized.  Such  rocks  are  known  in  the  gabbro 
area  of  the  Lizard9  (fig.  18),  and  in  Minnesota10  and  other 
American  districts  of  basic  intrusions. 

It  has  been  noticed  above  that  an  ordinary  gabbro  may 
pass  into  a  variety  very  rich  in  magnetite  and  ilmenite  (e.g. 

1  Watts,  Guide,  73. 

2  G.  H.  Williams,  Amer.  Geol.  (1890)  vi,  40-49,  pi.  n,  fig.  2. 

3  Merrill,  Proc.  U.  S.  Nat.  Mm.  (1894)  xvii,  662,  657,  658. 

4  Clements,  EuH.  No.  5,  Geol.  Sur.  Ala.  (1896)  163,  164. 

5  Turner,  A.  J.  S.  (1898)  v,  423,  424. 

6  Ferrier,  Ann.  Eep.  Geol.  Sur.  Can.  (1896)  viii,  344  L. 

7  Maskelyne,  Phil.  Map.  (1879)  vii,  135,  136. 

8  Bonney,  Q.  J.  G.  S.  (1878)  xxxiv,  778-780. 

9  Teall,  pi.  vin,  fig.  2. 

10  Wadsworth,  Prelim.  Descr.  Perid.  Gabb.  etc.  Minn.  (1887)  95,  pi.  v. 


86  IRON-ORE-GABBROS. 

Carrock  Fell).  Some  gabbros  and  norites,  in  Scandinavia,  in 
Minnesota1,  in  the  Adirondacks2,  etc.,  shew  very  basic  modifica- 
tions which  are  almost  pure  iron-ore-rocks3.  As  a  rule,  they 
are  highly  titaniferous.  An  augite-magnetite-rock,  consisting 
of  crystal-grains  of  augite  set  in  a  framework  of  titaniferous 
magnetite,  is  one  of  the  varieties  of  the  curious  banded 
gabbros  of  Skye4. 

1  Wadsworth,  ibid.  63,  64,  pi.  vi,  fig.  1;  Irving,  Copper-bearing  Rocks  of 
L.  Superior,  51,  52;   Winchell,  Wth  Ann.  Rep.  Minn.  Geol.  Sur.  (1882), 
80-83. 

2  Kemp,  Bull.  Geol.  Soc.  Amer.  (1894)  v,  222. 

3  Vogt,    G.  M.   1892,   82-86   (abstract).     For  descriptions   of  iron- 
ore-rocks  from  Cumberland  in  Rhode  Is.   and  Taberg  in  Sweden  see 
Wadsworth,  Butt.  Mus.   Comp.  Zool.  Harv.  (1881)  vii,  185-187;   Litli. 
Stud.  75-81,  pis.  i,  n. 

4  Geikie  and  Teall,  Q.  J.  G.  S.  (1894)  1,  pi,  xxvni. 


CHAPTER  VI. 


PERIDOTITES  (INCLUDING  SERPENTINE-ROCKS). 

THE  peridotites  are  holocrystalline  rocks  of  ultrabasic 
composition,  in  which  felspar  is  typically  absent  and  olivine  is 
the  most  prominent  constituent.  They  were  separated  from 
the  more  normal  basic  rocks  by  Rosenbusch ;  but,  though 
their  marked  characters  make  it  desirable  to  discuss  them 
apart,  they  do  not  constitute  a  family  comparable,  e.g.,  with 
that  of  the  gabbros  in  importance.  The  peridotites  do  not 
usually  occur  in  large  bodies  of  uniform  rock.  In  many 
localities  they  are  seen  to  be  only  local  modifications  of 
olivine-gabbros,  olivine-norites,  or  olivine-diorites,  and  they 
shew  frequent  transitions  from  one  type  to  another. 

For  so  small  a  group  a  needless  multiplicity  of  names  has 
been  created.  The  simple  olivine-rock  is  the  'dunite'  of 
Hochstetter.  With  the  addition  of  enstatite  we  have  the 
'  saxonite '  of  Wadsworth1,  '  harzburgite '  of  Rosenbusch  ;  other 
types  are  styled  '  Iherzolite,3  '  eulysite,'  etc.,  and  the  name 
'picrite'  is  used  for  those  characterized  by  augite  or  horn- 
blende, usually  with  some  felspar.  For  our  purposes  it  will 
be  sufficient  to  separate  the  picrites,  rich  in  the  bisilicate 
constituents  and  having  usually  subordinate  plagioclase,  from 
the  more  typical  peridotites,  very  rich  in  olivine  and  non- 
felspathic.  Different  types  may  be  specified  by  prefixes  in 
the  customary  way  (e.g.  hornblende-picrite,  enstatite-peridotite, 
etc.}. 

1  Lithological  Studies  (1884,  Camb.,  Mass.).  This  work  contains 
many  descriptions  of  peridotites  and  meteorites,  with  a  number  of 
useful  coloured  plates. 


88  MINERALS   OF   PERIDOTITES. 

Many  of  the  meteorites  ('stony  meteorites'  as  distin- 
guished from  meteoric  irons)  have  a  mineral  composition  allied 
to  that  of  the  terrestrial  peridotites,  but  often  with  special 
accessory  minerals  and  peculiar  structures1. 

In  consequence  of  the  unstable  nature  of  their  principal 
constituent  mineral,  the  peridotites  are  very  readily  decom- 
posed, and  most  of  the  serpentine-rocks  have  originated  in  this 
way. 

Constituent  minerals.  In  the  typical  peridotites 
olivine  makes  up  from  half  to  nearly  the  whole  of  the  rock. 
If  not  so  abundant  that  its  crystals  interfere  with  one 
another,  it  builds  idiomprphic  or  rounded  crystals.  The 
mineral  is  colourless  in  thin  slices,  and  shews  either  irregular 
cleavage-traces  or  a  network  of  fissures.  It  often  has  schiller- 
inclusions  of  the  nature  of  minute  negative  crystals  enclosing 
dendritic  growths  of  magnetite  (fig.  1,  h).  Alteration  along 
cracks  gives  rise  to  strings  of  magnetite  granules,  and  complete 
destruction  produces  pseudomorphs  of  greenish  or  yellow  ser- 
pentine, or  sometimes  colourless  fibrous  tremolite,  etc. 

Of  the  other  ferro-magnesian  silicates  the  commonest  in 
typical  peridotites  is  a  rhombic  pyroxene ;  either  colourless  or 
pale  yellow  (enstatite)  or  with  faint  green  and  rose  pleochroism 
(bronzite)  :  varieties  rich  in  iron  do  not  often  occur.  The 
crystals  often  tend  to  be  idiomorphic.  Any  marked  schiller- 
structures  are  not  very  common.  Decomposition  results  in 
pseudomorphs  of  bastite2.  The  augite  is  either  light  brown  to 
colourless,  with  a  high  extinction-angle  (about  40°)  as  in  many 
diabases,  etc.,  or  it  may  shew  a  faint  green  tint  (chrome- 
diopside).  A  conversion  to  brown  hornblende  is  common  in 
the  picrites,  and  so  also  are  parallel  growths  of  augite  and 
brown  hornblende,  the  former  being  the  kernel3. 

The  hornblende  may  be  a  green  or  pale  actinolitic  variety, 
but  in  many  of  the  picrites  it  is  *  basaltic '  hornblende  with  an 
extinction-angle  of  about  20°  and  colour  varying  from  deep 

1  See  Farrington,  Joitrn.   Geol.  (1901)  ix,  51-66,  174-190,  393-408, 
522-532. 

2  Fouqu6  and  Le"vy,  pis.  LIII,  LIV. 

3  Cohen  (3),  pi.  xxxn,  fig.  1. 


MINERALS  OF   PERIDOTITES. 


89 


brown  to  colourless.  The  pale  variety  seems  due  to  bleaching, 
often  accompanied  by  a  discharge  of  magnetite-dust.  The 
biotite  of  peridotites  also  is  frequently  of  a  pale  tint. 

Some  peridotites  have  little  octahedra  of  magnetite,  but 
some  other  spinellid  mineral  is  more  characteristic.  It  may 
be  chromite  (deep  brown  or  opaque),  picotite  (coffee-brown),  or 
pleonaste  (green).  These  minerals  usually  build  irregular 
rounded  grains.  In  some  of  the  rocks  perofskite  is  a  charac- 
teristic mineral,  in  minute  crystals1. 

A  basic  felspar  occurs  in  many  of  the  picrites,  but  is 
wholly  wanting  in  the  more  typical  peridotites.  Some  types 
have  accessory  garnet,  which  is  always  the  magnesian  variety 


FIG.  20. 


P(ECILITIC    STRUCTURE    IN    HORNBLENDE-PICRITE,    MYNYDD 

PENARFYNNYBD,  CAERNARVONSHIRE  ;    x  20. 


The  large  plate  enclosing  olivine-grains  and  filling  the  field  is  a  single 
crystal  of  hornblende.  It  is  mostly  colourless,  but  becomes  deep  brown 
in  capriciously  arranged  patches  round  the  edge  [725J. 

pyrope,  red-brown  in  slices.  Metallic  nickeliferous  iron  occurs 
in  some  of  the  meteoric  peridotites,  besides  special  minerals, 
such  as  troilite. 

1  Gf.  G.  H.  Williams  on  the  serpentine  of  Syracuse,  N.  Y.,  A,  J.  S. 
(1887)  xxxiv,  140-142: 


90 


STRUCTURES   OF   PERIDOTITES. 


Structure.  The  constituents  follow,  as  a  rule,  the  normal 
order  of  crystallization,  the  olivine  constantly  preceding  the 
bisilicates.  In  many  picrites,  and  in  other  types  not  too  rich 
in  olivine,  the  more  or  less  rounded  crystals  of  olivine  are 
enclosed  by  large  plates  of  pyroxene  or  hornblende  (jxecUitic 
structure1,  fig.  20).  When  felspar  occurs,  it  is  later  than  the 
pyroxenes,  but  in  the  hornblende-picrites  it  is  often  moulded 
in  ophitic  fashion  by  part  of  the  hornblende. 

In  the  most  basic  peridotites  the  largely  predominant 
olivine  builds  a  granular  aggregate,  in  which  may  be  im- 
bedded, with  a  pseudo-porphyritic  appearance,  relatively  large 


FIG.  21.     ENSTATITE-PEIUDOTITK  WITH  PSEUDO-PORPHYRITIC  STRUCTURE, 
SKUTVIK,  NEAR  TROMSO,  NORWAY  ;    x  20. 

Here  olivine  is  largely  in  excess,  forming  a  granular  aggregate  in 
which  are  embedded  large  irregular  crystals  of  a  yellowish  partly  altered 
enstatite  [440]. 

crystals  of  enstatite,  etc.  (fig.  21).  Any  true  porphyritic 
structure  (i.e.  some  constituent  occurring  in  two  distinct 
generations)  is  rare  in  this  family  of  rocks,  the  minerals 
usually  forming  an  even-grained  aggregate. 

1  This  is  quite  analogous  to  the  ophitic  structure  of  diabases,  etc. 
See  G.  H.  Williams,  A.  J.  8.  (1886)  xxxi,  30,  31 ;  Journ.  of  Geol.  (1893) 
i,  176. 


CELYPHITE-STRUCTURE   IN   PERIDOTITES.  91 

The  pyrope-bearing  peridotites  often  shew  a  special  type  of 
structure,  each  garnet-crystal  being  surrounded  by  a  broad 
border  or  shell  known  as  celyphite1  (Ger.  Kelyphit).  This 
border  is  sharply  divided  from  the  garnet,  and  possesses  a 
marked  radial  fibrous  structure.  The  name  is  not  applied  to 
any  particular  mineral,  and  the  so-called  celyphite  is  not 
always  of  the  same  constitution.  A  pale  or  colourless  augite 
is  common,  while  brown  hornblende  and  enstatite  are  some- 
times found,  and  brown  picptite  frequently  accompanies  the 
pyroxene.  Again,  brown  biotite  and  magnetite  have  been 
observed a.  A  celyphite-border  round  garnet  is  also  a  charac- 
teristic feature  in  pyroxene-garnet-rocks  (eclogites).  Some 
petrologists  have  regarded  it  as  a  secondary  'reaction-rim/ 
but  there  seems  to  be  no  decisive  reason  for  rejecting  the 
primary  origin  of  the  growth. 

Most  of  the  meteoric  peridotites  have  a  peculiar  structure 
termed  chondritic3.  A  fine-grained  matrix  of  olivine,  enstatite, 
chromite,  etc.,  encloses  numerous  round  grains  (chvndri)  con- 
sisting of  the  same  minerals.  In  these  chondri  the  crystals 
very  commonly  have  a  tendency  to  diverge  from  a  point  on 
the  circumference. 

Leading  types.  Numerous  examples  of  rocks  rich  in 
olivine  are  known  from  the  old  gneiss  area  of  Sutherland, 
from  the  western  islands  of  Scotland,  from  North  Wales, 
Cornwall,  etc.  There  are  frequent  transitions  from  felspar- 
bearing  picrites  to  thoroughly  ultrabasic  peridotites4. 

At  Penarfynnydd5,  on  the  south-west  coast  of  Caernarvon- 
shire, is  an  Ordovician  intrusion  ranging  from  Jiornblende- 
picrite  to  a  hornblende-peridotite  very  rich  in  olivine.  The 
hornblende  is  either  deep  brown  or  colourless,  in  the  same 

1  Bosenbusch-Iddings,  pi.  xiv,  fig.  4 ;  Cohen  (3),  pi.  LXI,  fig.  2. 

2  Diller,  A.  J.  S.  (1886)  xxxii,  123;    Bull.  No.  38,  U.  S.  Geol.  Sur. 
(1887)  15-17. 

3  For  figures  see  Wadsworth's  Lithological  Studies ;  Lockyer,  Nature 
(1890)  xli,  306,  307;  Farrington,  Journ.  Geol.  (1901)  ix,  174-180. 

4  For  figures  of  several  of  these  rocks,  see  Teall. 

5  Q.J.  G.S.  (1888)  xliv,  454-457.   Bala  Vole.  Rocks  of  Caern.  99-101. 
Similar  rocks  are  recorded  from  Breaker  Hill  and  Balhamie   Hill  in 
Ayrshire:    see   Mem.  Geol.   Sur.,   Silur.   Rocks   Scot.    (1899)    469,    470, 
pi.  xxin,  fig.  1. 


02  HORNBLENDE-PICRTTES. 

crystal,  and  it  encloses  the  rounded  grains  of  olivine  with 
typical  poecilitic  structure  (fig.  20).  A  colourless  augite 
and  a  deep  brown  biotite  occur,  with  a  little  original  mag- 
netite. Part  of  the  hornblende  is  formed  at  the  expense  of 
augite.  Anorthite  is  often  present,  usually  embraced  by  the 
hornblende.  Similar  rocks  occur  in  central  Anglesey,  where 
secondary  crystal-outgrowths  from  the  hornblende  are  fre- 
quent1. Prof.  Bonney3  has  described  some  of  these  rocks, 
which  occur  as  boulders  on  the  west  coast  of  Anglesey.  The 
same  writer  has  described  from  Sark:J  a  somewhat  different 
type  in  which  a  pale  altered  mica  is  a  prominent  mineral, 
besides  pale  or  greenish  actinolite.  This  seems  then  to  be  a 
mica-hornblende-picrite,  and  Prof.  Bonney  compares  it  with 
the^Scye  type  mentioned  below.  G.  H.  Williams4  has  given 
an  interesting  account  of  hornblende-picrites  from  the  Cort- 
landt  district  on  the  Hudson  River;  They  resemble  very 
closely  the  British  examples  and  a  well-known  rock  from 
Schriesheim,  near  Heidelberg5,  the  bleaching  of  the  brown 
hornblende  and  subordinate  brown  biotite  being  a  character- 
istic feature.  Examples  from  Alabama6  have  either  brown 
or  very  pale  green  hornblende,  and  contain  abundant  pleo- 
naste.  One  from  Montana7  has  accessory  hypers thene.  Prof. 
Bonney8  has  described  a  hornblende-picrite  from  Swift's  Creek, 
Gippsland,  Victoria. 

An  augite-picrite  of  Carboniferous  age  is  found  at  Inch- 
colm9,  near  Edinburgh,  in  which  the  dominant  coloured 
mineral  is  a  purplish-brown  pleochroic  augite,  often  with  hour- 
glass structure10.  Deep  brown  hornblende  is  also  present, 
chiefly  as  a  marginal  intergrowth  with  the  augite.  Felspar 

1  Teall,  pi.  vi. 

2  Q.  J.  G.  S,  (1881)  xxxvii,  187-140;  (1883)  xxxix,  254-259.     Also  a 
similar  rock  from  Alderney,  -ibid.  (1889)  xlv,  384. 

3  G.  M.  1889,  109-112. 

4  A.  J.  S.  (1886)  xxxi,  31-37  ;  and  in  Diller,  294-297. 

5  For  coloured  plate  see  Berwerth,  Lief.  in. 

8  Clements,  Bull.  No.  5,  Geol.  Sur.  Ala.  (1896)  155-160. 

7  Merrill,  Proc.  U.  S.  Nat.  Mm.  (1894)  xvii,  654. 

8  M.  M.  (1884)  vi,  54. 

9  Cole's  Stud.  Micro.  Sci.  (1882)  No.  6;  Teall,  pi.  iv,  fig.  2,  vn. 

10  The  'augite  resembles  that  of  some  nepheline-dolerites,  and  the  rock 
differs  in  other  respects  from  true  plutonic  types. 


AUGITE-   AND   ENSTATITE-PICRITES. 


93 


and  biotite  are  subordinate.  Most  of  the  olivine  is  con- 
verted into  a  yellow  serpentine.  Augite-picrites  with  typical 
pcecilitic  structure  occur  in  Shropshire1.  Among  examples 
from  the  Inner  Hebrides  Prof.  Judd2  notes  one  from  the 
Shiant  Isles  with  fine  poecilitic  structure.  Others  occur  in 
the  Cuillin  Hills  in  Skye.  Busz  has  described  an  augite- 
picrite  with  comparatively  fresh  olivine  from  Highweek,  near 
Newton  Bushel,  Devonshire  :  this  has  subordinate  enstatite 
and  biotite. 

Intrusions  of  enstatite-picrite  occur  in  the  old  gneiss  of  the 
west  of  Sutherland.  In  one  near  Lochinver  the  slightly 
pleochroic  enstatite  or  bronzite  is  moulded  on  the  olivine,  but 
shews  good  crystal-faces,  being  enclosed  by  large  crystal-plates 
of  felspar.  There  is  a  subordinate  colourless  augite  and  some 
brown  hornblende,  which  is  partly  formed  from  the  pyroxenes, 


•pi 


FIG.  22.     ENSTATITE-PEIUDOTITE,  ASSYNT  LODGE,  SUTHERLAND  ;    x  20. 

A  granular  aggregate  of  olivine  (o),  largely  serpentinized,  and  a 
slightly  pleochroic  enstatite  or  bronzite  (e).  These  two  minerals  are 
in  about  equal  quantity ;  in  addition  there  are  little  irregular  grains  of 
isotropic  green  pleonaste  (pi)  [1642]. 

1  Watts,  Rep.  Brit.  Ass.  for  1887,  700 ;  Proc.  GeoJ.  Ass.  (1894)  xiii, 
340,  fig. 

2  Q.  J.  G.  S.  (1885)  xli,  393,  pi.  xni,  tig.  4. 


94  MICA-    AND   HORNBLENDE-PERIDOTITES. 

partly  original  and  later  than  the  felspar.  This  rock  is  almost 
as  much  a  norite  as  a  picrite,  but  true  enstatite-peridotites 
also  occur  in  the  district,  consisting  of  about  equal  parts  of 
olivine  and  a  rhombic  pyroxene,  with  grains  of  pleonaste  (fig.  22). 

Of  mica-peridotite  few  examples  are  described.  One  from 
Elliott  County,  Kentucky1,  consists  of  serpentinized  olivine 
and  pale  yellow-brown  to  colourless  mica,  with  poecilitic 
arrangement,  besides  crystals  of  peroskite,  etc.  Sears2  records 
a  mica-peridotite  from  Andover  in  Massachusetts.  Prof. 
Judd3  has  described  under  the  name  '  scyelite '  a  hornblende- 
mica-peridotite  from  the  borders  of  Sutherland  and  Caithness 
(Loch  Scye  and  Achavarasdale  Moor).  Here  serpentinized 
grains  of  olivine  are  enclosed  in  poecilitic  fashion  by  a  pale 
green  to  colourless  hornblende,  probably  pseudomorphous 
after  diallage,  and  a  peculiar  yellow  mica.  A  similar  rock  is 
recorded  at  a  point  2j  miles  s.  E.  of  Borgie  Bridge.  Prof. 
Sollas  has  remarked  a  hornblende-hypersthene-peridotite  among 
the  crystalline  schists  of  Galway,  at  Derreennagusfoor.  This 
consists  of  hypersthene,  hornblende,  olivine,  magnetite,  and 
a  green  spinel.  Such  rocks  occur  also  in  Custer  County, 
Colorado4,  and  in  other  localities5. 

Among  hornblende-peridotites  we  may  place  the  rock  de- 
scribed as  a  hornblende-picrite  from  Greystones  in  Wicklow", 
which  is  non-felspathic.  The  dominant  hornblende  is  green, 
and  encloses  in  poecilitic  fashion  the  olivme-pseudomorphs  (of 
magnetite  and  a  carbonate).  It  has  cores  and  borders  of 
colourless  hornblende,  and  there  is  a  third  variety  of  this 
mineral  with  few  cleavage-cracks  and  much  magnetite-dust. 

Various  augite-peridotites  have  been  described.  Specimens 
of  these,  as  well  as  augite-picrites,  are  represented  among  the 
Tertiary  eruptives  of  western  Scotland7.  One  from  the  Isle 

1  Diller,  A.  J.  S.  (1892)  xliv,  286-289. 

2  Bull.  Essex  Inst.  (1894)  xxvi. 

3  Q.  J.  G.  S.  (1885)  xli,  401-407.     Teall,  pi.  v,  fig.  2. 

4  Cross,  Proc.  Colo.  Sci.  Soc.  (1887)  242-244. 

5  Manbhiim  in  India ;  see  Holland,  Eec.  Geol.  Sur.  Lid.  (1894)  xxvii, 
142 :  Kilimanjaro  ;  see  Hatch,  G.  M.  1888,  257-260. 

6  Watts,  Rep.  Brit.  Ass.  for  1893,  767 ;  Guide,  35. 

7  Judd,  Q.  J.  G.  S.  (1885)  xli,  389-395. 


AUGITE-   AND   ENSTATITE-PERIDOTlTES.  95 

of  Rum  shews  fresh  olivine  set  in  a  framework  of  green 
augite.  Magnetite  and  chromite  are  accessories,  and  some- 
times hypersthene.  Others  from  the  Cuillin  Hills,  Skye,  con- 
sist of  predominant  olivine,  brownish  diallage,  sometimes  a 
little  anorthite,  and  either  deep  brown  picotite  or  an  opaque 
mineral,  probably  chromite  or  chrome-magnetite.  Merrill 
has  described  an  augite-peridotite,  from  Little  -Deer  Island, 
Maine1,  and  a  diallage-mica-peridotite  from  Montana". 

A  well-known  enstatite-augite-peridotite  occurs  in  the  Pyre- 
nees and  Ariege  (Lherz  type)3.  About  two-thirds  of  the  rock 
consists  of  fresh  olivine,  the  other  minerals  being  a  colourless 
enstatite,  a  faint  green  to  colourless  chrome-bearing  diopside, 
and  irregular  grains  of  either  brown  picotite  or  green  pleo- 
naste.  As  usual  in  types  very  rich  in  olivine,  the  structure  is 
granular,  not  poecilitic.  Such  rocks,  often  serpentinized,  are 
recorded  from  several  other  districts.  A  porphyritic  bronzite- 
diallage-peridotite  occurs  in  Maryland4,  and  a  similar  rock  in 
Colusa  County,  California5. 

In  some  enstatite-peridotites  the  rhombic  pyroxene  is 
abundant,  and  forms  a  framework  in  which  the  somewhat 
rounded  grains  of  olivine  are  set  with  pcocilitic  structure. 
A  well-known  representative  comes  from  the  Harz  (Baste  or 
Harzburg  type)*5,  where,  however,  both  minerals  are  more  or 
less  completely  serpentinized.  A  similar  rock  is  described 
from  Presque  Isle,  Michigan7. 

In  another  type  olivine  largely  predominates,  and  the 
enstatite  occurs  in  relatively  large  crystals,  which,  among  the 
smaller  grains  of  olivine,  give  a  pseudo-porphyritic  appearance 
to  the  rock.  Good  examples  occur  near  Tromso,  etc.,  in 

Proc.  U.  8.  Nat.  Mm.  (1888)  xi,  192-1%. 
Ibid.  (1894)  xvii,  651,  652. 

See  Bonney,  G.  M.  1877,  59-64,  and  for  coloured  figures  Teall,  pi. 
i,  fig.  1 ;  Fonque  and  L6vy,  pi.  LII,  fig.  1. 

G.  H.  Williams,  Amer.  Geol.  (1890)  vi,  38,  39,  pi.  n,  fig.  1. 
Wadsworth,  Lith.  Stud.  pi.  v,  figs.  1-3. 

6  Wadsworth,  ibid.  133,  134,  pi.  vm,  figs.  1,  2,  5 ;  Fouque  and  Levy, 
pi.  LIII,  fig.  2. 

7  Wadsworth,  Lith.  Stud.  136-138,  pi.  vn,  figs.  3-5. 


96  OLI VINE-ROCK  :    ANORTHITE-PERIDOTITES. 

Norway  (fig.  21),  Inyo  County,  California1,  etc.2  In  Maryland, 
Williams'1  has  described  similar  rocks  in  which  large  crystals 
of  bronzite  or  diallage,  or  both,  are  embedded  in  a  granular 
mass,  mainly  of  olivine.  In  Montana4  occurs  a  peridotite 
with  abundant  crystals  of  bronzite  and  olivine  enclosed  in  a 
finely  granular  aggregate  of  enstatite,  pale  hornblende,  some 
olivine,  and  green  pleonaste. 

From  these  rocks  it  is  only  a  step  to  one  composed  wholly 
of  olivine,  with  only  a  little  accessory  picotite  or  magnetite. 
Of  this  pure  divine-rock  the  type  comes  from  New  Zealand 
(Mount  Dun),  and  is  the  'dimite'  of  Hochstetter.  In  Skye 
very  beautiful  examples  come  from  the  southern  part  of  the 
Cuillin  Hills,  near  Loch  Scavaig.  Here  the  only  mineral  in 
addition  to  olivine  is  one  of  the  spinellid  group,  usually  deep 
brown  picotite  but  sometimes  green  pleonaste.  The  picotite 
is  in  good  octahedra,  and  in  certain  narrow  seams  becomes 
the  principal  element.  In  America  examples  of  dunite  come 
from  Franklin,  Webster5,  and  Corundum  Hill",  all  in  North 
Carolina,  and  from  Western  Massachusetts7. 

We  may  class  with  the  peridotites  certain  oli vine-felspar- 
rocks  which  differ  from  the  troctolites  mentioned  in  the  last 
chapter  in  having  more  abundant  olivine  and  constantly 
anorthite  as  the  felspathic  element.  Such  anorthite-peridotites 
occur  in  Skye  constantly  associated  with  picrites,  and  bear 
the  same  relation  to  these  that  the  troctolites  do  to  olivine- 
gabbros.  Here  we  may  place  rocks  described  by  Prof.  Judd8 
from  Alival  in  the  Isle  of  Rum  and  by  Prof.  Bonney9  from 
Belhelvie  in  Aberdeenshire. 


1  Wadsworth,  Lith.  Stud.  132,  pi.  vi,  fig.  4. 

3  One  from  New  Zealand  carries  grains  of  nickel-iron  alloy  (awaruite), 
Ulrich,  Q.  J.  G.  S.  (1890)  xlvi,  625-629,  pi.  xxiv. 

3  Bull.  No.  28,  U.  S.  Gcol.  Sur.  (1886)  50-55  •  Amcr.  Geol.  (1890)  vi,  38, 
39,  pi.  ii,  fig.  1. 

4  Merrill,  Proc.  U.  S.  Nat.  Mm.  (1894)  xvii,  655. 

5  Wadsworth,  Lith.  Stud.  118,  119,  pi.  iv,  figs.  2,  3. 

6  Chatard,  Bull.  No.  42,  U.  S.  Geol.  Sur.  (1887)  45. 

7  Martin,  A.  J.  S.  (1893)  vi,  244-247. 

8  Q.  J.  G.  S.  (1885)  xli,  pi.  xin,  fig.  5. 

9  G.  M.  (1885)  441,  442. 


SERPENTINE-ROCKS.  97 

Of  garnet- peridotites  that  from  Elliott  County,  Kentucky1, 
is  a  good  example.  The  pyrope  crystals  are  surrounded  by 
a  'celyphite'  border  of  brown  mica  with  an  outer  ring  of 
magnetite-dust,  these  minerals  being  supposed  to  be  due  to  a 
reaction  between  the  garnet  and  the  olivine.  The  serpentine- 
rock  of  Zoblitz  in  Saxony  is  another  example,  in  which, 
however,  the  olivine  is  wholly  destroyed.  Garnet  occurs  as 
an  accessory  in  the  diallage-peridotite  of  Tunaberg  in  Norway2 
(the  '  eulysite '  of  Erdmann)  and  in  other  localities. 

Serpentine-rocks.  Hitherto  we  have  noticed  only  very 
briefly  the  secondary  changes  that  affect  the  minerals  of 
crystalline  rocks.  In  the  present  family,  however,  the  De- 
composition of  a  rock  is  often  so  complete  that  its  original 
nature  is  detected  only  by  careful  study,  and  the  altered  rock- 
masses  are  commonly  denoted  by  a  special  name,  serpentine- 
rocks  or  simply  serpentines,  expressing  their  dominant  mineral 
composition.  The  mineral  serpentine  is  the  commonest  de- 
composition-product of  the  non-aluminous  magnesian  silicates 
(olivine,  the  rhombic  pyroxenes,  and  some  of  the  augites  and 
hornblendes),  and  the  purest  serpentine-rocks  result  from  the 
alteration  of  peridotites3.  Other  decomposition-products  occur 
in  the  rocks,  viz.  iron- oxides  (magnetite  and  limonite),  steatite, 
carbonates  (dolomite,  etc.),  chlorite,  and  tremolite;  but  the 
bulk  is  serpentine  of  various  kinds,  in  which  may  be  found 
undestroyed  relics  of  the  original  minerals  of  the  peridotite 
(olivine,  diopside,  pyrope,  chromite,  etc.). 

Of  the  mineral  serpentine  some  kinds  are  crystalline  and 
doubly  refracting,  with  the  interference- colours  of  quartz  or 
felspar  and  faint  pleochroism  when  the  green  tint  is 
sufficiently  pronounced.  The  habit  is  fibrous  (chrysotile)  or 
scaly  (antigorite,  etc.).  Other  kinds  are  amorphous  and 

1  Diller,  290-294,  pi.  xxxix ;  A.  J.  S.  (1886)  xxxii,  121-125 ;  Bull.  38 
U.  S.  Geol.  Sur.  (1887).      . 

2  Wadsworth,  Lith.  Stud.  147. 

3  For  descriptions  of  coloured  figures  of  numerous  serpentine-rocks, 
see  Wadsworth,  Lithological  Studies  (1884).    For  a  general  sketch  of 
observations  and  opinions  on   serpentine,  see    Teall,   Chap.   vi.      On 
serpentine  from  diopside,  see  Merrill,  Proc.  U.  S.  Nat.  Mus.  (1888)  xi, 
105-109,  pi.  xxxii ;  A.  J.  S.  (1889)  xxxvi,  189-191. 


H.   P. 


98  MICRO-STRUCTURES   OF  SERPENTINE. 

sensibly  isotropic.  Much  of  the  serpentine  occurs  in  definite 
pseudomorphs,  and  often  retains  something  of  the  structure 
of  the  parent  mineral  to  indicate  its  source.  We  may  dis- 
tinguish four  cases : 

(i)  Serpentine  derived  from  olivine,  with  the  '  mesh- 
structure1'  (Tschermak's  '  Maschenstructur ' ;  see  p.  74  and 
fig.  23). 

(ii)  Serpentine  derived  from  enstatite  or  bronzite,  in 
distinct  pseudomorphs  with  the  bastite-structure  (see  p.  73 
and  fig.  23). 

(iii)  Serpentine  derived  from  a  non-aluminous  hornblende, 
with  '  lattice-structure2'  (' Gitterstructur '  of  Weigand).  Here 
the  cleavage  of  the  hornblende  is  marked  by  veins  of  birefring- 
ent  serpentine  in  two  sets  making  the  characteristic  angle 
55  J°.  This  serpentine  is  minutely  fibrous,  with  the  fibres  set 
perpendicularly  to  the  cleavage  of  the  hornblende.  The  rest 
of  the  pseudomorph  is  of  serpentine  giving  no  definite  crystal- 
line reaction  and  consisting  probably  of  a  confusedly  fibrous 
aggregate. 

(iv)  Serpentine  derived  from  a  non-aluminous  augite,  with 
'knitted-structure'33  ('gestrickte  Structur'  of  Hussak).  This 
consists  chiefly  of  serpentine  with  scaly  habit  (antigorite). 
The  scales  give  straight  extinction  and  low  polarization-tints. 
They  occur  in  two  closely  interlacing  sets  parallel  to  the 
cleavage-planes  of  the  augite,  and  so  making  an  angle  of  about 
87°  with  one  another. 

The  derivation  of  serpentine  from  pyroxene  is  very  clearly 
exhibited  in  some  American  occurrences  described  by  Merrill 
at  Montville,  N.J.4  and  in  Essex  County5  and  Warren 
County6,  N.Y. 

The  source  of  serpentine  in  rocks  can  often  be  made  out 
by  these  various  characters,  and  it  is  placed  beyond  doubt 
when  any  unaltered  remnants  of  the  parent  mineral  remain. 

1  Cohen  (3),  pi.  LXII,  fig.  1. 

2  Ibid.  fig.  3.  3  Ibid.  fig.  4. 

4  Proc.  U.  S.  Nat.  Mus.  (1888)  xi,  105-111,  pi.  xxxn. 

5  Ibid.  (1889)  xii,  595-599. 

6  A.J.S.  (1889)  xxxvii,  189-191. 


CORNISH   SERPENTINES.  99 

In  addition  there  may  be  serpentine  encroaching  upon  con- 
tiguous minerals  or  traversing  them  in  veins :  this  is,  as  a  rule, 
sensibly  isotropic. 


FIG.  23.     SERPENTINE-BOCK,  COVERACK,  CORNWALL  ;    x  20. 

A  large  bastite-pseudomorph  after  bronzite  is  seen  on  the  right. 
The  rest  of  the  rock  is  of  serpentine  with  mesh-structure,  derived  from 
olivine:  it  is  stained  in  places  with  hydrated  iron-oxide  [1118]. 

The  best-known  serpentine-rocks  in  this  country  are  those 
of  the  Lizard  district  in  Cornwall1.  The  purer  examples 
consist  essentially  of  serpentine  of  various  kinds,  secondary 
iron-ore  (often  peroxidized),  steatite,  tremolite,  etc.,  and  often 
undestroyed  relics  of  olivine  or  other  original  minerals  of 
the  peridotites.  Professor  Bonney  has  shewn  that  much  of 
the  serpentine  has  the  character  of  that  derived  from  olivine, 
and  some  of  the  original  rocks  were  probably  nearly  pure 
olivine-rocks  (Dun  type).  Others  were  enstatite-  or  bronzite- 
peridotites,  and  shew  large  bastite-pseudomorphs  after  a 
rhombic  pyroxene  (Cadgwith,  Coverack,  etc. ;  fig.  23,  cf.  fig. 
2 1)2.  Others  again  are  altered  hornblende-peridotites,  some 
of  the  serpentine  shewing  the  mesh-  and  some  the  lattice- 

1  Bonney,  Q.  J.  G.  S.  (1877)  xxxiii,  915-923 ;  and  (1883)  xxxix,  21-23; 
Teall,  115  et  seqq. 

2  See  also  Teall,  pi.  i,  tig.  2 ;  Cole's  Micro.  Stud.  (1883)  No.  50. 

7—2 


100  CORNISH   AND   OTHER   SERPENTINES. 

structure,  while  relics  of  olivine,  hornblende,  and  picotite  may 
remain  (Mullion  Cove,  Kynance  Cove,  etc.)1.  Augite-picrites 
are  also  represented  (Menheniot,  etc.).  Here  felspar  has  been 
altered  into  a  substance  resembling  serpentine,  which  Mr  Teall 
thinks  is  probably  that  called  pseudophite.  Tremolite  has 
been  formed  at  the  expense  of  olivine.  The  augite  of  the 
original  rock  is  often  preserved.  Prof.  Bonney  and  Gen. 
McMahon2,  summarising  the  features  of  the  Lizard  serpentines, 
say  that  they  "can  be  roughly  separated  into  two  groups  :  in 
the  one  a  foliated  mineral  of  the  enstatite  group  is  a  conspicu- 
ous accessory ;  in  the  other  a  colourless  augite  or  hornblende, 
usually  the  latter.  A  few  are  non-porphyritic3,  and  in  some 
cases  exhibit  no  certain  traces  of  any  pyroxenic  mineral, 
rhombic  or  monoclinic,  though  of  course  a  spinellid  or  some 
iron  oxide  is  always  to  be  detected,  and  in  one  instance  (at 
the  Rill4,  W.  of  Kynance  Cove) 'the  presence  of  a  fair  propor- 
tion of  felspar  has  been  asserted." 

Various  serpentinous  rocks  are  found  near  Holyhead  and 
in  neighbouring  parts  of  Anglesey.  That  of  Ty-ucha  is 
regarded  by  Prof.  Bonney5  as  an  altered  olivine-rock.  In 
rocks  at  Four-mile  Bridge  much  of  the  serpentine  has  the 
character  of  that  derived  from  augite,  and  the  parent-rock 
seems  to  have  been  genetically  connected  with  a  gabbro  mass. 
Mr  Blake,  however,  finds  indications  of  olivine-  and  enstatite- 
serpentine6. 

Of  the  numerous  serpentine-rocks  of  Scotland7,  one  at 
Balhamie  Hill  in  Ayrshire  has  been  described  by  Prof.  Bonney8 
as  an  altered  olivine-bronzite-rock,  closely  resembling  that  of 
Cadgwith  in  Cornwall,  the  structure  being  of  the  pseudo- 


1  See  Teall,  pi.  xv. 

2  Q.  J.  G.  S.  (1891)  xlvii,  466. 

3  In  the  sense  of  containing  no  conspicuous  crystals. 

4  Teall,  p.  119.     "  The  original  rock,  therefore,  was  of  the  nature  of 
a  picrite. "     See  also  G.  M.  1887,  137,  138. 

5  Q.  J.  G.  S.  (1881)  xxxvii,  45. 

6  Rep.  Brit.  Ass.  for  1888,  408. 

7  For  coloured  plate  of  Portsoy  serpentine  see   Cole's  Micro.  Stud. 
No.  52. 

a  Q.  J.  G.  S.  (1878)  xxxiv,  770. 


AMERICAN  SERPENTINES.  101 

porphyritic  type.  Some  near  Belhelvie  in  Aberdeen  shire l 
have  also  been  enstatite-peridotites,  but  with  the  p03cilitic 
structure,  and  now  shew  pseudomorphs  after  olivine  set  in  a 
framework  of  bastite,  just  as  in  the  rock  of  Baste  in  the 
Harz,  which  has  given  its  name  to  the  latter  mineral. 

Numerous  serpentine-rocks  are  known  in  the  United  States. 
Wadsworth  has  described  peridotite-serpentines  from  Min- 
nesota2, from  Plumas  County,  California,  from  Westfield 
and  Ly nnfield,  Mass. ,  and  other  localities 3.  Near  San  Francisco 
occur  some  derived  from  peridotites  (the  Potrero4),  others 
from  pyroxene-rocks  (Angel  Island5).  The  rock  at  Syracuse, 
N.Y.,  was  shewn  by  Williams6  to  be  an  altered  peridotite, 
sharply  denned  pseudomorphs  after  olivine  and  enstatite  being 
easily  detected,  while  the  remaining  constituents,  viz.  brown 
mica,  perofskite,  and  chromite,  are  still  preserved. 

1  Bonney,  G.  M.  1885,  440. 

2  Prelim.  Descr.  Perid.  Gabb.  etc.  Minn.  (1887)  29,  pi.  i,  fig.  1. 

3  Lithological  Studies  (1884),  158-160,  pi.  vi,  tigs.  2,  5,  VH,  fig.  2. 

4  Palache,  Bull.  Geol.  Dep.  Univ.  Gal.  (1894)  i,  165-169. 

5  Ransome,  ibid.  (1894)  i,  220-222. 

6  A.  J.  S.   (1887)  xxxiv,  140-142.     For  other  serpentines  from  New 
York  State  see  Newland,  School  of  Mines  Quarterly  (1901),  xxii,  307-317, 
399-410. 


B.     HYPABYSSAL    ROCKS. 


SOME  petrologists  are  content  to  divide  the  igneous  rocks 
into  two  great  groups,  according  as  their  structural  characters 
indicate  consolidation  under  deep-seated  or  under  superficial 
conditions.  Others,  however,  recognize  another  group  inter- 
mediate between  these  two.  Thus  Kosenbusch  inserts  between 
his  '  Tiefengesteine '  and  'Ergussgesteine '  a  group  'Gang- 
gesteine'  or  'dyke-rocks.'  The  rocks  to  be  treated  under  the 
present  head  correspond  in  a  general  way,  though  not  precisely, 
with  the  last  named,  but  Brogger's  name  'hypabyssal'  is 
adopted  as  more  accurately  expressing  the  characters  upon 
which  the  group  is  founded. 

Although  this  threefold  division  seems  to  be  necessitated 
by  a  comparative  study  of  the  great  variety  of  rock-types  met 
with  in  nature,  it  must  be  admitted  that  the  hypabyssal  group 
is  a  somewhat  artificial  one,  the  rocks  included  in  it  lacking 
any  well  defined  set  of  common  characteristics  distinguishing 
them  from  the  other  two  groups.  Any  definition  would  have 
to  be  framed  chiefly  in  negative  terms,  and  would  bring 
together  types  presenting  many  points  of  difference  from  one 
another.  Most  of  them  are  holocrystalline,  but  in  some  a 
glassy  residue  is  found.  In  some  families  the  porphyritic 
structure  is  characteristic1,  as  it  is  in  the  volcanic  rocks ;  in 
others  it  is  wanting  or  non-significant :  but  even  the  holo- 
crystalline non-porphyritic  types  have  structural  and  mineral- 
ogical  characters,  to  be  noted  below,  which  differentiate  them 
from  rocks  of  truly  deep-seated  origin. 

1  On  the  significance  of  this  structure  see  Cross,  14M  Ann.  Rep.  U.  S. 
GeoL  Sitr.  (1895)  232-235  ;  Pirsson,  A.  J.  S.  (1899)  vii,  271-280  ;  Crosby, 
Amer.  GeoL  (1900)  xxv,  299. 


CHAPTER  VII. 

ACID  INTRUSIVES. 

THE  acid  hypabyssal  rocks  embrace  a  considerable  range  of 
varieties,  bridging  over  the  difference  between  the  even-grained, 
holocrystalline  granites  and  the  porphyritic,  largely  glassy 
rhyolites.  The  porphyritic  character  is  almost  universal,  but 
the  ground-mass  which  encloses  the  phenocrysts  may  be  holo- 
crystalline, partly  crystalline  and  partly  glassy,  or  wholly 
glassy.  On  the  nature  and  special  structures  of  the  ground- 
mass  depend  chiefly  the  several  types  usually  recognized  among 
these  rocks.  All  agree  in  that  the  constituent  minerals — in  so 
far  as  these  are  developed — include  in  the  first  rank  felspars 
rich  in  alkali  and  usually  quartz,  while  ferro-magnesian  minerals 
and  free  iron-ores  occur  only  in  relatively  small  quantity,  and 
are  sometimes  wanting. 

From  an  examination  of  their  mineral  constitution  and 
characteristic  structures,  the  more  crystalline  types  are  readily 
referred  to  their  proper  positions ;  but,  in  proportion  as  the 
bulk  of  the  rock  comes  to  consist  of  unindividualised  glassy 
matter  or  an  irresolvable  cryptocrystalline  'base,'  the  criteria 
become  fewer.  In  particular,  the  first  stage  of  consolidation 
(that  of  the  phenocrysts)  may  have  been  arrested  before  quartz 
(the  last  mineral)  began  to  crystallize,  and  so,  if  the  ground- 
mass  consolidates  as  a  glass,  we  may  have  a  thoroughly  acid 
rock  without  quartz.  Thus  the  most  glassy  rocks  (pitchstones) 
belonging  to  this  family  are  not  always  to  be  distinguished  by 
the  microscope  alone  from  less  acid  pitchstones.  Again,  they 
are  scarcely  divided  from  some  glassy  rhyolites  (obsidians). 


104  MINERALS   OF  THE   ACID  INTRUSIVES. 

The  nomenclature  of  the  acid  intrusives  is  confused.  The 
name  'felsite'  or — if  containing  evident  phenocrysts  of  quartz — 
'quartz-felsite'  has  been  applied  in  this  country  not  only  to  these 
rocks  but  also  to  many  volcanic  rocks  (acid  and  intermediate), 
and  its  usage  lacks  precision  and  significance.  The  name 
quartz-porphyry,  borrowed  from  the  German,  covers  most  of 
the  rocks,  but  not  all,  since  porphyritic  quartz  may  be  wanting : 
this  term  is  also  used  by  Continental  writers  for  the  '  older ' 
acid  lavas.  For  a  type  rich  in  soda,  and  having  some  miner- 
alogical  peculiarities,  the  name  quartz-ceratophyre  (Ger.  Quarz- 
keratophyr)  has  been  used.  It  will  be  convenient  to  speak 
of  the  family,  as  a  whole,  as  the  acid  intrusives.  The  names 
applied  to  particular  types  will  be  noticed  in  connection  with 
the  ground-mass. 

Constituent  minerals.  We  notice  here  especially  the 
minerals  occurring  as  phenocrysts.  Of  these,  the  felspars 
include  orthoclase  (not  microcline)  and  an  acid  plagioclase  such 
as  oligodase.  The  'two  are  commonly  associated,  and  both 
build  idiomorphic  crystals  with  the  usual  types  of  twinning. 
A  narrow  zone  of  orthoclase  surrounding  each  plagioclase 
crystal  is  seen  in  some  rocks.  The  characteristic  felspar  of 
the  quartz-ceratophyres  is  anorthoclase. 

The  quartz  has  crystallized  in  the  ordinary  hexagonal 
pyramids,  sometimes  with  narrow  prism-faces,  but  the  crystals 
are  frequently  rounded  and  eaten  into,  owing  to  corrosion  by 
the  ground-mass,  and  may  have  lost  all  crystal  outlines.  In 
the  rock-types  most  nearly  approaching  granites  (granite- 
porphyries)  the  quartz  contains  fluid-pores  :  in  other  types  the 
inclusions  are  mostly  of  glass  or  portions  of  the  ground-mass 
(fig.  24,  A).  As  already  mentioned,  quartz-phenocrysts  are 
not  always  present. 

The  brown  biotite,  which  occurs  in  many  of  the  rocks,  has 
the  same  characters  as  in  granites,  and  carries  the  same 
inclusions.  It  is  usually  in  good  hexagonal  flakes.  Less 
commonly,  in  the  marginal  part  of  an  intrusion,  it  has  a 
blade-like  habit,  due  to  extension  along  the  a-axis.  The  usual 
mode  of  alteration  is  chloritization1.  Hexagonal  flakes  of 
muscovite  are  found  in  a  few  of  the  granite-porphyries  only. 

1  Cohen  (3),  pi;  LVH. 


MINERALS   OF  THE   ACID   INTRUSIVES. 


105 


A  green  hornblende  in  well-built  crystals  is  a  rather  excep- 
tional constituent.  The  deep  blue  soda-bearing  amphibole 
riebeckite  occurs  in  a  few  rocks,  always  in  very  ragged  allo- 
triomorphic  crystals  (fig.  24,  .5).  The  augite  of  the  acid 


?;&&• 


msf 


B 


f*   ;•   vr  •;>  f*.-. 

nX^-ilrr 


f^Kip35 

lSilS^'S$ 

^gytiMM?^ 


w 


•> ," 

% 

^j???ll& 


FIG.  24.  x  20. 

J.  Quartz -porphyry,  dyke,  King's  Cove,  Arran.  The  quartz-pheno- 
crysts  are  partly  corroded,  and  contain  inclusions  of  the  ground-mass,  as 
well  as  relatively  large  glass-cavities  (gc)  with  the  form  of  '  negative 
crystals '  [3151]. 

7?.  Eiebeckite-Microgranite  ( '  paisanite  ' ),  Mynydd  Mawr,  Caernar- 
vonshire. The  nearly  opaque  crystals  of  sponge-like  form  are  the  dark 
blue  soda-amphibole,  riebeckite  [2750]. 

intrusives  is  a  pale  greenish  variety  like  that  in  some  granites, 
but  occurs  here  much  more  frequently.  It  builds  good  idio- 
morphic  crystals  in  many  granophyres  and  pitchstones.  A  few 
rocks  rich  in  soda  contain  ccgirme.  A  rhombic  pyroxene 
(bronzite)  is  also  known. 

As  accessories,  apatite  and  zircon  are  widely  but  sparingly 
distributed,  while  the  iron-ores  are  usually  represented  only 
by  a  little  magnetite.  Such  minerals  as  garnet,  tourmaline, 
and  pinite  pseudomorphs  after  cordierite1  occur  in  special 

1  Fouque  and  Levy,  pi.  xm,  fig.  5. 


106  STRUCTURES  OF  THE   ACID  INTRUSIVES. 

localities.     Some  granite-porphyries  carry  tourmaline  (Corn- 
wall;  Elba). 

Ground-mass  and  structures.  The  types  which  ap- 
proach most  nearly  to  the  plutonic  habit  are  known  as  granite- 
porphyry.  Here  relatively  large  idiomorphic  crystals  of  quartz 
and  felspars,  with  mica  or  some  other  ferro-magnesian  mineral, 
are  enclosed  in  a  fine-textured  crystalline  ground-mass  of 
felspar  and  quartz.  The  structure  of  this  ground  may  resemble 
that  of  a  granite,  or  may  be  distinguished  by  a  more  marked 
idiomorphism  of  the  lath-shaped  felspars,  usually  untwinned. 
Mica  may  also  occur  in  a  second  generation  as  part  of  the 
ground-mass. 

Very  common  are  the  types  in  which  the  phenocrysts, 
consisting  of  felspars,  more  or  less  corroded  quartz,  and  biotite 
or  some  other  constituent,  are  embedded  in  a  very  finely 
crystalline  ground-mass  of  felspar  and  quartz.  The  elements 
of  the  ground-mass  may  have  more  or  less  idiomorphism. 
Quartz-porphyries  having  an  evidently  microcrystalline  ground- 
mass  of  this  kind  are  styled  by  Rosenbusch  microgrcmites,  the 
porphyritic  character  being  understood1. 

When  the  texture  of  the  ground-mass  sinks  to  such  minute- 
ness as  to  be  not  clearly  resolved  under  the  microscope,  it 
may  be  described  as  cryptocrystalline  ('microfelsitic'  of  some 
authors).  For  such  rocks  Rosenbusch  uses  the  term  felso- 
phyre2.  Without  entering  into  a  discussion  of  an  obscure 
subject,  it  may  be  said  that  this  cryptocrystalline  ground  is 
probably  in  some  cases  original,  in  other  cases  due  to  secondary 
change  (devitrification)  of  a  ground-mass  originally  glassy. 

The  glassy  (or  '  vitrophyric ')  type  of  ground-mass  is  seen 
in  the  rocks  known  as  pitchstones.  In  some  of  these,  pheno- 
crysts of  felspar,  etc.,  are  only  sparingly  present,  the  great 
bulk  of  the  rock  consisting  essentially  of  isotropic  glass.  This 
glassy  ground,  however,  includes  in  many  cases  innumerable 
minute  and  imperfectly  developed  crystalline  growths  (crystal' 

1  For   chromolithograph   of    a   microgranitic   quartz-porphyry   from 
Halle,  see  Berwerth,  Lief,  i,  and  of  a  granite-porphyry  from  the  Oden- 
wald  Lief.  iv. 

2  Cf.  Teall,  G.  M.  1885,  108-111. 


MICROGRAPHIC  STRUCTURES.          107 

lites)  with  regular  grouping  (fig.  27).  These  minute  bodies  will 
be  more  fully  noticed  in  connection  with  the  acid  lavas.  The 
pitchstones  frequently  shew  perlitic  cracks,  and  occasionally 
some  of  the  flow-phenomena  which  are  better  exhibited  in 
lavas.  Typical  pitchstones,  excluding  lava-flows,  are  of  quite 
limited  distribution. 

In  the  above  types  we  have  what  may  be  regarded  as  a 
graduated  transition  from  the  granitic  to  the  rhyolitic  struct- 
ures, the  only  gap,  that  between  cryptocrystalline  matter  and 
glass,  being  one  which  the  instruments  at  our  disposal  do  not 
enable  us  to  bridge.  There  is,  however,  a  second,  more  or  less 
distinct,  line  of  transition,  parallel  to  the  former  but  charac- 
terized by  a  different  set  of  structures,  viz.  micrographic 
intergrowths  of  felspar  and  quartz  and  regular  crystalline 
aggregates  of  felspar  fibres.  To  these  structures  Rosenbusch 
applies  the  somewhat  inappropriate  term  '  granophyric,'  in- 
cluding both  micropegmatitic  and  microspherulitic :  and  the 
rocks  having  a  ground-mass  of  this  nature  are  very  generally 
known  as  granophyres. 

We  have  already  noticed  in  some  granites  a  micrographic 
intergrowth  of  the  kind  named  micropegmatite ;  but  when 
the  whole  mass  of  the  rock,  exclusive  of  crystals  of  certain 
minerals,  takes  on  this  character,  we  have  a  type  characteristic 
of  hypabyssal  rather  than  abyssal  rocks  as  here  understood. 
In  such  rocks  the  quartz  and  the  greater  part  of  the  felspar 
form  a  micrographic  ground-mass,  which  may  enclose  idio- 
morphic  crystals  of  some  ferro-magnesian  mineral  (augite  or 
biotite)  or  of  felspar  (mostly  plagipclase).  Further,  the  micro- 
graphic  intergrowth  may  come  in  to  some  extent  in  rocks 
which  on  the  whole  would  be  placed  with  the  granite-porphyries 
or  the  microgranitic  type.  When  the  intergrowth  is  on  a 
relatively  coarse  scale,  it  is  often  rude  and  irregular,  but  the 
finer-textured  '  micropegmatite '  shews  great  regularity  and 
often  a  definite  arrangement1  (fig.  25,  A\  In  particular  it 
frequently  forms  a  regular  frame  surrounding  phenocrysts  of 
felspar2,  and  it  can  often  be  verified  that  the  felspar  of  the 

1  Cohen  (3),  pi.  xxxni. 

2  For  good  illustrations  see  Irving,  Copper-bearing  Rocks,  L.  Superior, 
pi.  xiv,  tigs.  1,  2. 


108  MICROGKAPHIC  STRUCTURES. 

intergrowth  is  in  crystalline  continuity  with  the  felspar 
crystal  which  served  as  a  nucleus  (fig.  25,  B).  The  appear- 
ance is  as  if  the  original  crystal  had  continued  to  grow 


B 


FIG.  25.     GRANOPHYRES,  SHEWING  MICROGRAPHIC  INTERGROWXH  OF 
FELSPAR  AND  QUARTZ  ;    X    20,  CROSSED  NICOLS. 

A.  Crug,  near  Caernarvon  :  shewing  an  intricate  aggregate  of  rather 
delicate  micropegmatite  with  a  tendency  to  irregular  '  centric '  arrange- 
ment [17].  B.  Carrock  Fell,  Cumberland :  shewing  part  of  a  phenocryst 
of  oligoclase  with  a  fringe  of  micropegmatite.  The  felspar  in  this  is  in 
crystalline  continuity  with  the  phenocryst ;  the  quartz,  shewn  in  the 
position  of  extinction,  is  continuous  with  a  quartz-grain  at  the  top  of  the 
figure  [1545]. 

throughout  the  final  consolidation  of  the  rock,  enclosing  the 
residual  excess  of  silica  as  intergrown  quartz.  Sometimes  a 
line  of  Carlsbad  twinning  can  be  traced  from  the  crystal 
through  the  surrounding  frame.  There  is  no  doubt  that 
pkgioclase  felspar,  as  well  as  orthoclase,  enters  into  such 
micrographic  intergrowths.  Less  frequently  the  quartz  of  the 
intergrowth  is  seen  to  be  in  crystalline  continuity  with  a 
quartz  crystal  or  grain,  upon  which  it  has  grown. 

The  finest  micrographic   intergrowth   tends  especially  to 
a  stellate  or  radiate  ('  centric ')  arrangement,  with  or  without 


SPHERULITIC   STRUCTURES.  109 

a  nucleus  of  an  earlier  crystal.  As  the  growth  becomes  very 
delicate  in  texture,  the  sectors  within  which  the  felspar 
extinguishes  simultaneously  become  narrower,  and  are  repre- 
sented between  crossed  nicols  by  dark  rays  when  their 
direction  makes  a  small  angle  with  one  of  the  cross-wires. 
When  the  structure  is  on  too  minute  a  scale  to  be  resolved  by 
the  microscope,  it  may  be  termed,  by  analogy,  cryptographic. 
The  optical  characters  of  such  an  aggregate  appear  to  be 
determined  by  the  minute  radially  arranged  fibres  of  felspar, 
which  obscure  the  quartz.  The  structures  known  as  micro- 
splierulitic  and  pseudo-spherulitic  in  acid  rocks  are  probably 
of  this  nature  (fig.  26).  Between  crossed  nicols  they  shew 


FIG.  26.    GRANOPHYRE,  GLAS-BHEINN  BHEAG,  SKYE  ;   x   20, 

CROSSED   NICOLS. 

The  spherulites  (pseudospherulites  of  some  authors)  consist  of  a 
cryptographic  intergrowth  of  felspar  and  quartz  arranged  radially  about 
centres.  At  the  periphery  of  each  spherulite  the  cryptographic  passes 
into  a  visibly  micrographic  structure,  and  the  radial  arrangement 
becomes  less  marked.  Between  the  spherulites  are  interspaces  in  which 
the  structure  is  granular  [2494]. 

characteristically  a  black  cross,  caused  by  extinction  in  those 
fibres  which  lie  nearly  parallel  to  one  of  the  cross-wires.  Such 
growths  cluster  round  porphyritic  crystals  of  quartz  or  felspar, 


110  CORNISH   ELVANS. 

or,  as  innumerable  closely  packed  minute  spherules,  constitute 
almost  the  whole  of  the  ground-mass1. 

Isolated  spherulites  or  bands  of  spherulites  may  occur  in 
a  vitreous  or  devitrified  ground. 

Leading  types.  We  proceed  to  select  a  few  examples 
illustrating  the  several  points  indicated  above.  In  view  of 
the  frequent  association  of  the  different  types  of  ground-mass 
in  one  district  or  even  in  parts  of  one  intrusion,  we  shall  not 
find  it  convenient  to  follow  any  strict  order. 

The  Carboniferous  'elvan'  dykes  of  Cornwall  and  Devon, 
as  described  by  Mr  J.  A.  Phillips 2  and  by  Mr  Teall,  have 
a  microcrystalline  to  cryptocrystalline  ground-mass  enclosing 
large  felspars,  pyramidal  or  rounded  quartz  crystals,  and  often 
mica.  The  quartz  contains  either  glass-inclusions3  or  fluid- 
pores,  sometimes  in  the  form  of  negative  crystals4,  which  may 
enclose  a  salt-cube  as  well  as  a  bubble5.  Tourmaline  is  of 
frequent  occurrence  in  crystals  or  stellate  groups  of  needles, 
and  is  sometimes  seen  to  replace  felspar.  An  occasional 
constituent  is  cordierite,  represented  by  the  so-called  *  pinite ' 
pseudomorphs  of  yellowish  green  micaceous  flakes  (Sydney 
Cove6). 

The  varied  group  of  Ordoyician  intrusive  rocks  in  Caernar- 
vonshire7 include  some  granite-porphyries  of  a  well-marked 
type.  Quartz  is  wanting  among  the  phenocrysts,  which  are 
chiefly  of  oligoclase.  One  example  at  the  head  of  Nant 
Ffrancon  has  a  ground-mass  of  allotriomorphic  quartz  and 
felspar  (chiefly  orthoclase).  The  ferro-rnagnesian  constituent 

1  For  good  figures  of  micrographic  and  cryptographic  structures, 
ranging  from  the  micropegmatitic  to  the  spherulitic,  see   Fouqu6   and 
Le"vy,   pi.  x,  fig.  2 ;    xi,  fig.  1  ;    xn,  xiv,  xv,  xvi.      For   cryptographic 
structure  see  also  Cohen  (3),  pi.  xxxiv,  figs.  3,  4,  and  chromolith.   in 
Berwerth,   Lief,   iv    (from    Baden,   compare    micrographic    rock    from 
Vosges). 

2  Q.  J.  G.  S.  (1875)  xxxi,  334-338,  pi.  xvi ;  cf.  Cohen  (3),  pi.  xxn, 

fig.  1. 

a  Cohen  (3),  pi.  ix,  fig.  1. 

4  Ibid.  pi.  xi,  fig.  4. 

5  Ibid.  pi.  xn,  fig.  4. 

6  Teall,  334. 

7  Bala  Vole.  Ser.  Caern.  (1889)  48-56. 


WELSH   GRANITE-PORPHYRIES,   ETC.  Ill 

is  biotite.  Others,  quarried  at  Yr  Eifl  and  near  Nevin,  have 
a  ground  in  which  idiomorphic  felspars  are  moulded  by  inter- 
stitial quartz.  These  contain  augite,  usually  without  biotite. 
Other  rocks  in  the  district,  all  augitic,  shew  more  or  less 
tendency  to  micrographic  structures,  and  in  many  the  whole 
ground-mass  is  of  micropegmatite.  Beautiful  examples  occur 
in  the  hills  above  Aber  and  at  Moel  Perfedd  in  Nant 
Ffrancon.  The  growth  of  the  micropegmatite  round  felspar 
crystals  is  well  exhibited,  and  in  some  cases  a  narrow  zone  of 
orthoclase  is  seen  interposed  between  a  plagioclase  crystal  and 
the  surrounding  growth.  The  structure  is  rarely  so  minute 
as  to  approximate  to  the  spherulitic.  Many  of  the  smaller 
intrusions  in  the  district,  e.g.  near  Clynog-fawr,  are  of  quartz- 
porphyry  with  a  cryptocrystalline  ground,  which  may  possibly 
be  due  to  devitrification.  Porphyritic  quartz,  which  is  wanting 
in  the  more  evidently  crystalline  types,  appears  here  in 
corroded  crystal-grains.  A  somewhat  similar  rock  is  that 
forming  a  low  range  in  the  neighbourhood  of  Llanberis.  This 
exhibits  flow- structure  in  places,  and  has  been  considered  by 
Professor  Bonney  and  others  as  a  group  of  lavas. 

The  complex  group  of  acid  rocks  near  Caernarvon  and 
eastward,  which  some  have  supposed  to  be  of  pre-Cambrian 
age,  affords  examples  of  granite-porphyries,  micrographic  rocks 
(fig.  25,  A),  micrpcrystalline  and  spherulitic  quartz:porphyries, 
etc.  The  spherulitic  growths  often  surround  pyramids  of  quartz. 
The  porphyritic  felspars  in  all  these  rocks  are  mostly  plagio- 
clase, and  the  ferro-magnesian  mineral  is  biotite,  often  green 
from  alteration.  Various  granophyres  and,  especially,  beautiful 
spherulitic  rocks,  shewing  the  growth  round  pyramidal  crystals 
of  quartz,  occur  at  St  David's1.  The  structure  is  of  the  crypto- 
graphic type,  not  shewing  a  very  perfect  black  cross. 

The  Lake  District  contains  examples  of  microgranites, 
such  as  the  rock  quarried  at  Threlkeld,  while  some  minor 
intrusions  shew  a  cryptocrystalline  ground.  Granophyres  also 
occur,  the  large  Buttermere  and  Ennerdale  intrusion  being  of 
a  micropegmatitic  rock  with  either  biotite  or  augite,  resembling 
some  Caernarvonshire  examples.  The  dykes  of  Armboth  and 

1  Geikie,  Q.  J.  G.  S.  (1883)  xxxix,  315,  pi.  x,  figs.  8,  9. 


112  LAKE   DISTRICT  GRANOPHYRES,  ETC. 

Helvellyn  have  a  spherulitic  ground-mass  enclosing  idiomorphic 
crystals  of  quartz  and  felspar.  The  spherulitic  growth,  which 
does  not  always  give  a  good  black  cross,  is  clustered  especially 
about  the  quartz  crystals.  A  few  garnets  occur.  These  rocks 
are  probably  all  Ordovician.  The  Devonian  dykes  about 
Shap,  in  Edenside,  near  Sedbergh,  etc.,  have  microcrystalline 
to  cryptocrystalline  grounds,  and  some  of  them  contain  biotite 
rather  abundantly.  An  intrusion  near  Dufton  Pike1  in  West- 
morland is  a  characteristic  granite-porphyry  with  white  and 
dark  micas,  which  occur  both  as  phenocrysts  and  in  the 
ground-mass.  The  other  phenocrysts  are  idiomorphic  quartz 
and  felspar,  chiefly  plagioclase  but  with  a  few  large  sanidine- 
crystals.  A  marginal  modification  of  the  rock  shews  a  blade- 
like  habit  of  the  biotite,  a  peculiarity  found  also  in  some 
other  rocks  in  Westmorland  and  Cumberland,  such  as  the 
quartz-porphyries  of  Wansfell  and  Potter  Fell  and  the  borders 
of  the  Buttermere  and  Carrock  Fell  granophyres. 

One  of  the  most  beautiful  granophyres  in  this  country  is 
that  of  Carrock  Fell,  in  Cumberland2.  It  contains  a  pale 
augite  in  good  crystals,  often  uralitized  or  otherwise  altered, 
and  rarely  a  little  biotite.  There  are  also  idiomorphic  felspars, 
usually  oligoclase,  and  some  granules  of  iron-ore.  The  ground- 
mass  shews  in  different  specimens,  or  even  in  one  slide,  every 
gradation,  from  a  coarse  irregular  micropegmatite  through 
exquisitely  regular  micrographic3  and  cryptographic  structures 
to  what  would  be  described  as  spherulitic.  These  inter- 
growths  usually  make  up  the  whole  ground-mass,  though 
sometimes  part  of  the  quartz  forms  irregular  grains.  The 
arrangement  is  sometimes  '  centric,'  but  more  usually  peripheral 
to  the  felspar  phenocrysts,  forming  a  regular  border  to  them. 
It  can  often  be  seen  that  the  felspar  of  the  intergrowth  is 
continuous  with  that  of  the  crystal,  and  much  of  it  must  be 
plagioclase  (fig.  25,  B). 

Augite-  and  hornblende-granophyres  figure  largely  among 
the  Tertiary  intrusions  of  Scotland  and  Ireland,  e.g.  in  Skye, 

1  Q.  J.  G.  8.  (1891)  xlvii,  519. 

2  Ibid.  (1895)  li,  126-129. 

3  Teall,  pi.  XLVII,  fig.  5  (misplaced  4  in  key-plate). 


SCOTTISH   AND   IRISH   GRANOPHYRES,  ETC.  113 

Mull1,  and  the  Carlingford  district.  Some  are  quite  coarse 
micropegmatites,  or  shew  only  a  rude  kind  of  intergrowth, 
and  these  rocks  are  frequently  miarolitic.  The  more  delicate 
micrographic  and  cryptographic  growths  are,  however,  repre- 
sented (fig.  26).  One  variety,  at  Meall  Dearg  in  Skye,  has 
riebeckite  instead  of  augite2.  A  remarkable  rock  from  Corrie- 
gills  in  Arran3  appears  as  if  divided  into  polygonal  areas, 
each  enclosing  a  spherule  with  well-marked  boundary  and 
radial  structure.  Dr  Hyland4  has  described  granophyre 
dykes  in  Co.  Down.  These  contain  apparently  no  augite, 
but  a  little  green  hornblende  (Newcastle)  or  brown  mica 
(Hilltown). 

The  biotite-bearing  quartz-porphyries  of  the  Cheviots5  have 
sometimes  granophyric  structures,  but  are  more  commonly 
micro-  to  cryptocrystalline.  Frequently  the  ground-mass  en- 
closes patches  of  micropegmatite  like  porphyritic  crystals,  some- 
times shewing  the  outlines  of  idiomorphic  felspar.  This  feature 
is  also  well  shewn  in  a  microgranitic  quartz-porphyry  from 
the  Black  Hill  in  the  Pentlands6.  Among  Scottish  quartz- 
porphyries  of  Tertiary  age  those  which  form  numerous  sills 
and  dykes  in  the  Isle  of  Arran  are  worthy  of  notice.  The 
ground-mass  is  microcrystalline  in  the  larger  intrusive  bodies 
but  often  cryptocrystalline  in  the  smaller  (fig.  24,  A). 

More  interesting  are  the  well-known  and  beautiful  Arran 
pitchstones7,  of  which  some  are  of  acid,  others  of  subacid  com- 
position. They  form  dykes,  probably  of  Tertiary  age.  The 
phenocrysts  are  of  sanidine,  quartz,  plagioclase,  and  augite, 
varying  in  different  examples  and  sometimes  occurring  very 
sparingly.  The  ground-mass  is  of  glass  crowded  with  crystal- 


1  Teall,  pi.  xxxm,  fig.  1. 

2  Teall,  Q.  J.  G.  S.  (1894)  1,  219. 

3  Allport,  G.  M.  1872,  541  ;  Bonney,  G.  M.  1877,  506-508. 

4  Sci.  Proc.  Roy.  Dubl.  Soc.  (1890)  vi,  420-430. 

5  Teall,  G.  M.  1885,  111  ;  Kynaston,  Tr.  Edin.  G.  S.  (1899)  vii,  402- 
408,  pi.  xxv,  figs.  2,  3. 

6  Flett,  ibid.  483-486,  pi.  xxvn,  figs.  2-4. 

7  Allport,  G.  M.  1872,  1-9  ;  1881,  438  ;  Bonney,  G.  M.  1877,  499-511 ; 
Judd,  Q.  J.  G.  S.  (1893)  xlix,  546-551,  559-561,  pi.  xix ;  Teall,  pi.  xxxiv, 
figs.  3,  4 ;  Cohen  (3),  pi.  iv,  fig.  1. 

H.   P.  8 


114 


ARRAN   PITCHSTONES. 


lites,  which  often  assume  peculiar  groupings.  In  one  variety 
needle-shaped  microlites  (belonites)  of  hornblende  occur,  each 
forming  the  trunk  of  a  delicate  arborescent  aggregate  of  more 
minute  bodies  (Corriegills,  fig  27,  A  and  B).  In  another 


B 


FIG.  27.    PITCHSTONES,  AKKAN. 
Arborescent  crystallites  with  stellate  grouping,  Corriegills  ;    x 


A. 
[2752]. 

B.  The  same ;    x  100. 

C.  Plumose  crystallites  with  cross-like  grouping ; 


20 


100  [2751]. 


variety  occur  crosses,  each  of  the  four  arms  carrying  a  plume- 
like  growth  (Tormore,  fig.  27,  C).  Again,  little  rod-like  bodies 
frequently  occur  as  a  fringe  arranged  perpendicularly  on  the 
faces  of  phenocrysts.  The  general  mass  of  the  glass  is  full  of 
very  minute  crystallitic  bodies,  but  around  each  grouping  is  a 
clear  space,  indicating  that  the  tree-like  or  other  growth  has 
been  built  up  at  the  expense  of  the  surrounding  part.  Flow- 
structures  are  only  occasionally  met  with,  and  perlitic  cracks 
are  not  common.  Dykes  of  pitchstone  with  various  crystallitic 
growths  occur  also  in  Skye  (Glamaig,  near  Sligachan,  and 
Coirechatachan  near  Broadford)  and  in  Donegal  (Barnesmore 


AMERICAN   QUARTZ-PORPHYRIES,   ETC.  115 

Gap)1.  All  these  British  pitchstones  are  remarkable  for  their 
richness  in  ferro-magnesian  crystallites,  sometimes  of  horn- 
blende, sometimes  of  augite. 

Acid  intrusives  rich  in  soda  (quartz-ceratophyres)  are  not 
yet  well  known  in  this  country.  Probably  some  of  the 
'soda-felsites'  of  Leinster2,  of  Ordovician  age,  are  to  be  placed 
here.  They  are  microcrystalline  rocks,  with  or  without 
porphyritic  structure,  consisting  essentially  of  predominating 
felspar  and  quartz.  Plagioclase  is  much  more  abundant  than 
orthoclase,  and  is  sometimes  albite,  sometimes  possibly  anor- 
thoclase  or  cryptoperthite. 

Among  American  examples  may  be  mentioned  the  horn- 
blende-granite-porphyries described  by  Zirkel3  for  the  Fortieth 
Parallel  Survey.  These  carry  porphyritic  quartz  and  felspars, 
plagioclase  being  prominent,  hornblende,  biotite,  and  often 
sphene,  with  a  microcrystalline  ground-mass.  The  quartz  has 
fluid-pores  sometimes  containing  salt-cubes  and  other  inclu- 
sions4. Typical  examples  occur  at  Franklin  Buttes,  Nevada, 
in  the  Oquirrh  Mts,  Utah,  etc.  Rocks  with  cryptocrystalline 
ground-mass  ('  felsite-porphyry ')  also  occur,  though  in  less 
force5,  and  spherulitic  varieties  are  found  (Spruce  Mt,  Peoquop 
Range).  A  granite-porphyry  similar  to  the  above  has  been 
described  in  detail  by  Iddings6  from  the  Eureka  district, 
Nevada ;  and  Pirsson7  has  given  an  account  of  granite- 
porphyries,  some  with  biotite,  others  with  hornblende,  from 
the  Little  Belt  Mts,  Montana.  Quartz-porphyries  carrying 
tourmaline  occur  in  the  Castle  Mountain  district,  Montana8, 
and  in  the  Tintic  Mts,  Utah9.  The  quartz-porphyries  of  the 
Black  Hills,  S.  Dakota,  have  been  described  by  J.  D.  Irving10. 
Few  typical  pitchstones  have  been  recorded  in  the  United 


1  Sollas,  Sci.  Pr.  Eoy.  Dubl.  Soc.  (1893)  viii,  87-91. 

2  Hatch,  G.  M.  1889,  70-73,  545-549. 

3  Micro.  Petrogr.  Fortieth  Parallel  (1876),  62-67. 

4  Ibid.  63,  77,  pi.  i,  fig.  5.  5  Ibid.  73-80. 

6  Monog.  xx  U.  S.  Geol.  Sur.  (1893)  339-345. 

7  20th  Ann.  Eep.  U.  S.  Geol.  Sur.  (1900)  part  m,  498-511. 

8  Weed  and  Pirsson,  Bull.  139  U.  S.  Geol.  Sur.  (1896)  99-103. 

9  Tower  and  Smith,  19th  Ann.  Eep.  U.  S.  Geol.  Siir.  (1899)  part  m,  632. 
10  Ann.  N.  Y.  Acad.  Sci.  (1899)  xii,  276-281. 

8—2 


116  RIEBECKITE-QUARTZ-PORPHYRIES. 

States.  Osann1  notices  one  from  the  Eagle  Mts  in  Western 
Texas,  which  recalls  the  Arran  rocks,  containing  stellate 
groupings  of  green  augite  crystallites. 

In  America  also  a  number  of  anorthoclase-bearing  rocks 
have  been  described  which  fall  into  this  family.  We  may  note 
especially  some  of  the  pre-Cambrian  granophyres,  passing  into 
granites  (soda-granite)  in  the  Lake  Superior  region  and  else- 
where. Such  rocks  have  been  described  and  figured  by 
Irving2  and  more  particularly  by  Bayley3  (Pigeon  Point, 
Minn.). 

More  remarkable  are  those  rocks  in  which  the  ferro- 
magnesian  minerals  are  also  of  soda-bearing  varieties.  From 
the  Apache  Mts,  in  Western  Texas,  Osann4  has  described  a 
riebeckite-granite-porphyry  (Paisani  type),  having  scattered 
porphyritic  felspars  (microperthite)  and  quartz  in  a  ground- 
mass  containing  ragged  crystal-grains  of  riebeckite  with 
microperthitic  felspar  and  quartz.  Washington5  notes  from 
Magnolia  Point,  Mass.,  a  rock  with  a  fine-textured  ground- 
mass  containing  minute  needles  of  greenish-blue  glaucophane- 
riebeckite ;  and  in  another  from  Bass  Rocks  in  the  same 
district  the  coloured  silicate  is  deep  blue  glaucophane6. 
Allied  to  the  Paisani  rock  is  that  of  Mynydd  Mawr  in 
Caernarvonshire7  (fig.  24,  _#)  which  has  ragged  crystals  of 
riebeckite  with  microperthitic  felspars  and  some  quartz-grains 
in  a  ground-mass  of  quartz  and  felspar  with  microlites  of 
some  unknown  mineral.  A  somewhat  similar  rock  occurs  at 
Ailsa  Craig8,  and  the  occurrence  of  riebeckite  in  a  Skye 
granophyre  has  been  mentioned  above. 

In  other  cases  the  ferro-magnesian  is  a  soda-pyroxene. 
Brogger  has  described  in  the  Christiania  district  an  segirine- 

1  4th  Ann.  Rep.  Geol.  Sur.  Tex.  (1892)  134. 
'2  Copper-bearing  Rocks  of  L.  Superior,  pi.  xv. 

3  Bull.  No.  109  U.  S.  Geol.  Sur.  (1893). 

4  4M  Ann.  Rep.  Geol.  Sur.  Tex.  (1893)  131,  132. 
6  Journ.  Geol.  (1899)  vii,  111-113. 

6  Ibid.  117,  118. 

7  G.  M.  1888,    225,  226;    1889,   455,  456;  Bala  Vole.  Ser.  Caern. 
(1889)  50-52. 

8  Teall,  M.  M.  (1891)  ix,  219-221 ;  Heddle,  Tr.  Edin.  G.  S.  (1897) 
vii,  266,  pi.  xv,  fig.  1. 


^EGIRINE-QU  ARTZ-PORPH  YRIES.  117 

granite-porphyry  in  which  the  characteristic  mineral  occurs 
both  as  phenocrysts  and  abundantly  in  the  ground-mass 
(Grorud  type).  Rocks  more  or  less  closely  comparable  with 
this  are  found  in  the  Black  Hills  of  Dakota1  and  at  Judith 
Peak  in  Montana8.  Varieties  approximating  to  the  same 
type  occur  near  Inchnadampf  in  Sutherland.  Mr  Teall3 
describes  one  as  consisting  of  poly  synthetic  aggregates  re- 
presenting original  phenocrysts  of  alkali  felspar,  streaks 
of  microcrystalline  quartz  (scarce),  and  a  crypto-  or  micro- 
crystalline  felspathic  matrix  crowded  with  acicular  microlites 
of  segirine.  A  rock  of  coarser  texture  and  richer  in  the 
pyroxenic  element  comes  from  the  remote  islet  of  Rockall 
in  the  Altantic.  It  consists  simply  of  aegirine,  albite,  and 
quartz4. 

1  J.  D.  Irving,  Ann.  N.  Y.  Acad.  Sci.  (1899)  xii,  248-257. 

2  Pirsson,  18th.  Ann.  Rep.  U.  S.  GeoL  Sur.  (1898)  part  in,  558,  559. 

3  G.  M.  1900,  391. 

4  Judd,  2V.  Roy.  Ir.  Acad.  (1897)  xxxi,  part  in,  48-58,  pi.  xii. 


CHAPTER  VIII. 

PORPHYRIES  AND   PORPHYRITES. 

THE  rocks  which  are  for  convenience  grouped  together  in 
this  chapter  belong  to  various  hypabyssal  types  of  intermediate 
chemical  composition.  They  have  not  a  very  wide  distribu- 
tion, and  they  graduate  on  the  one  hand  into  the  acid 
intrusives  already  discussed,  on  the  other  into  the  more 
peculiar  family  of  the  lamprophyres. 

The  porphyritic  structure  characterizes  almost  all  the 
rocks  in  question,  and  in  most  of  the  types  is  marked  by 
felspar  phenocrysts  of  relatively  large  size.  The  ferro- 
magnesian  minerals  are  often  confined  to  the  elements  of 
the  earlier  period  of  crystallization.  Original  quartz  is  found 
in  the  more  acid  types  only,  and  is  almost  always  restricted 
to  the  ground-mass. 

The  rocks  may  be  regarded  as  standing  between  the 
plutonic  syenites,  diorites,  etc.,  on  the  one  hand,  and  the 
volcanic  trachytes,  dacites,  and  andesites  on  the  other,  just 
as  the  rocks  treated  in  the  preceding  chapter  stand  between 
the  granites  and  the  rhyolites.  According  as  the  dominant 
constituent  is  an  alkali-felspar  or  a  soda-lime-felspar,  they 
fall  into  two  families,  to  be  distinguished  as  porphyries  and 
porphyrites  respectively. 

Under  the  former  head  we  may  recognize  syenite-porphyry 
and  orthoclase-porphyry  (or  simply  porphyry),  corresponding 
with  granite-prophyry  and  quartz-porphyry  among  the  acid 
rocks.  From  these  orthoclase-bearing  rocks  have  been  separ- 
ated others  characterized  by  a  potash-soda-felspar,  under  the 


MINERALS  OF  PORPHYRIES  AND  PORPHYRITES.   119 

name  ceratophyre  (Ger.  Keratophyr).  There  are  also  nepheline- 
syenite-porphyry  and  nepheline-porphyry(tinguaite,  etc.),  which 
are  of  very  restricted  occurrence. 

Of  the  rocks  characterized  by  soda-lime-felspars,  the  types 
most  nearly  approaching  the  plutonic  have  been  styled  diorite- 
porpkyrite,  etc.,  the  others  being  termed  simply  porphyrites. 
Since  some  ferro-magnesian  mineral  is  usually  a  prominent 
constituent,  we  have  the  divisions  miccuporphyrite,  hornblende- 
porphyrite,  and  augite-porphyrite.  If  a  little  porphyritic 
quartz  be  present,  we  have  a  quartz-porphyrite  (quartz-mica- 
porphyrite). 

It  must  be  noted  that  writers  who  make  no  distinction  in 
nomenclature  between  intrusive  and  volcanic  rock-types  use 
some  of  the  above  names  in  a  more  extended  sense.  Thus  the 
Continental  petrologists  include  under  the  term  porphyrite 
the  'older'  andesitic  lavas,  while  some  British  authors  apply 
the  same  name  to  andesites  modified  by  secondary  changes 
(partial  decomposition,  etc.).  Some  of  the  altered  rocks  styled 
propylites  belong  to  the  division  now  to  be  considered,  others 
being  lavas. 

Constituent  minerals.  The  orthoclase  phenocrysts  of 
the  porphyries  are  similar  to  those  in  the  quartz-porphyries  and 
other  acid  intrusives.  In  the  porphyrites  this  mineral  does 
not  occur  except  in  the  ground-mass.  A  plagioclase  felspar 
accompanies  the  porphyritic  orthoclase  in  some  of  the  por- 
phyries, and  forms  the  most  conspicuous  phenocrysts  in  the 
porphyrites.  Here  it  builds  idiomorphic  or  rather  rounded 
crystals,  with  twinning  often  on  two  or  three  different  laws. 
It  ranges  in  the  porphyrites  from  oligoclase  to  labradorite, 
and  frequently  shews  strong  zoning  between  crossed  nicols. 
A  parallel  intergrowth  of  orthoclase  and  plagioclase  is  common 
in  some  porphyries.  In  certain  types  of  that  family  also 
occurs  a  felspar  which  has  been  referred  to  anorthoclase,  while 
it  has  also  been  explained  as  a  minute  parallel  intergrowth  of 
a  potash-  and  a  soda-lime-felspar.  Viewed  between  crossed 
nicols,  a  crystal  is  often  seen  to  be  divided  rather  irregularly 
into  portions  with  different  optical  behaviour,  sometimes  one 
part  finely  striated,  another  without  visible  striation.  In 
certain  special  rocks  (rhomb-porphyries)  the  crystal  has  a 


120   STRUCTURES   OF   PORPHYRIES   AND   PORPHYRITES. 

peculiar  habit,  which  gives  a  lozenge-shaped  section;  in  the 
ceratophyres  it  has  the  usual  habit,  giving  rectangular 
sections. 

As  phenocrysts  quartz  is  found  only  sparingly  in  a  few 
rocks,  but  it  enters  into  the  ground-mass  of  all  the  more  acid 
of  the  porphyries  and  porphyrites,  though  less  abundantly 
than  in  the  true  acid  rocks. 

The  most  usual  ferro-magnesian  minerals  are  brown  biotite 
and  a  pale  or  colourless  idiomorphic  augite.  Some  of  the 
porphyrites  have  hornblende  in  sharply  idiomorphic  prisms, 
often  twinned:  it  is  more  usually  brown  than  green.  In 
rocks  rich  in  alkali  the  coloured  constituent  is  often  segirine- 
augite  or 'true  wgirine. 

As  accessories,  apatite  and  iron-ores  (often  titaniferous) 
may  occur  in  varying  quantity,  the  latter  not  being  abundant. 
Exceptionally  olivine  and  other  minerals  are  present. 

In  the  few  rocks  which  contain  nepheline  that  mineral 
occurs  in  one  or  two  generations.  As  phenocrysts  it  is 
idiomorphic,  while  the  little  crystals  in  the  ground-mass 
may  or  may  not  have  definite  shape.  The  '  liebenerite ' 
pseudomorphs  in  certain  porphyries  have  been  supposed  to 
represent  nepheline.  They  consist  essentially  of  a  pale  mica, 
and  may  with  equal  probability  come  from  the  destruction 
of  cordierite.  Some  of  these  rocks  rich  in  alkali  carry  melanite 
garnet. 

Ground-mass  and  Structures.  In  the  great  ma- 
jority of  the  rocks  here  considered  the  ground-mass  is 
holocrystalline,  with  a  fine  texture  and  with  various  types 
of  structure.  It  consists  essentially  of  felspar  or,  in  the  more 
acid  members,  of  felspar  and  quartz.  In  the  porphyries  the 
felspar  is  usually  in  minute  prisms,  short  in  comparison  with 
their  length,  and  as  a  rule  untwinned.  Quartz,  if  present, 
occurs  interstitially.  The  little  prisms  may  have  more  or  less 
of  a  parallel  arrangement,  due  to  flow.  Such  short  and 
relatively  stout  prisms  are  usually  referred  to  orthoclase  :  if 
the  crystals  have  the  '  lath '-shape,  they  are  probably  of  a 
plagioclastic  variety.  Any  approach  to  an  allotriomorphic 
character  is  uncommon,  and  the  micrographic  intergrowths  so 


SYENITE-POKPHYRIES.  121 

frequent  among  the  acid  intrusives  are  not  found  here.  In 
the  nepheline-bearing  rocks  a  more  allotriomorphic  type  of 
structure  is  often  found ;  while  the  bostonites  and  allied 
rocks  shew  an  approach  to  the  volcanic  trachytes,  often  with 
marked  flow-structure. 

The  ground-mass  of  the  porphyrites  is  also  in  general 
holocrystalline,  consisting  essentially  of  felspar,  or,  in  the 
more  acid  varieties,  of  felspar  and  quartz.  In  this  latter  case 
the  rocks  may  reproduce  some  of  the  characteristic  structures 
noted  in  the  preceding  chapter,  such  as  the  cryptecrystalline 
and  the  micrographic.  Other  porphyrites  have  the  'ortho- 
phyric '  type  of  ground-mass  (with  short  felspar-prisms),  as  in 
the  porphyries,  but  there  is  every  gradation  from  this  to  the 
allotriomorphic.  In  some  of  the  more  basic  members  the 
ground-mass  consists  of  little  lath-shaped  plagioclase  prisms 
with  more  or  less  noticeable  flow-arrangement,  an  approach  to 
the  character  of  some  andesites  ('  pilotaxitic '  structure). 

Glassy  and  vitrophyric  rocks  are  not  unknown  in  the 
families  in  question.  Some  of  the  Arran  pitchstones,  for 
example,  have  the  composition  of  intermediate  rather  than 
acid  rocks. 

Leading  types.  Only  a  few  illustrative  examples  will 
be  selected,  chiefly  from  British  and  American  rocks. 

As  an  example  of  syenite-porphyry  may  be  noticed  the  rock 
quarried  at  Enderby  in  Leicestershire.  It  contains  phenocrysts 
of  a  strongly  zoned  plagioclase  felspar  and  of  pale  greenish 
brown  hornblende  with,  more  sparingly,  flakes  of  biotite  and 
round  grains  of  quartz  in  a  moderately  fine-textured  ground- 
mass  of  quartz  and  felspar,  apparently  orthoclase. 

Syenite-porphyries  in  considerable  variety  have  been  de- 
scribed from  the  United  States.  Some  with  hornblende 
occur  in  the  Little  Belt  Mts,  Mont.1  From  Cape  Ann,  Mass., 
Washington2  describes  dykes  of  quartz-syenite-porphyry,  in 
which  the  coloured  silicates  are  green  hornblende  and 
subordinate  biotite.  This  rock  he  compares  with  Brogger's 

1  Pirsson,  20th  Ann.  Rep.  U.  S.  Geol.  Sur.  (1900)  part  in,  513-515. 

2  Journ.  Geol.  (1899)  vii,  108,  109. 


122  jEGIRINE-SYENITE-PORPHYRIES. 

Lindo  type,  from  the  Christiania  district,  which,  however,  is 
typically  non-porphyritic.  A  rock  from  Coney  Island,  Salem, 
Mass.,  has  abundant  phenocrysts  of  felspar  (microperthite  and 
cryptoperthite)  in  a  ground-mass  of  similar  felspars  and  needles 
of  a  greenish  blue  soda-amphibole  (catophorite),  with  fluxion- 
structure.  Augite-syenite-porphyry  has  been  noted  at  Lake 
Chatauqua,  N.Y.,  Albany,  N.H.  (with  accessory  bronzite), 
and  other  places. 

Richer  in  alkali  is  the  segirine- syenite-porphyry  (Solvsberg 
type)  described  by  Brogger  in  the  Christiania  district.  Here 
the  structure  of  the  ground-mass  approaches  that  seen  in  the 
trachytic  lavas  and  in  the  bostonites  noticed  below.  A  similar 
rock  is  that  described  (under  the  name  '  acmite-trachyte ')  by 
Wolff  and  Tarr1  from  the  Crazy  Mts  in  Montana.  The  pheno- 
crysts are  of  anorthpclase  and  augite  (bordered  by  segirine) 
with  occasional  sodalite,  and  the  ground-mass  is  of  lath-shaped 
felspars  (chiefly  anorthoclase)  and  needles  of  segirine,  with  a 
variable  amount  of  nepheline  and  secondary  analcime.  Rocks 
more  or  less  comparable  with  this  occur  near  Manchester  in 
Massachusetts2  and  in  the  Apache  Mts  of  Texas.  From  the 
Sierra  Nevada  of  California  Turner3  has  described  a  'soda- 
syenite-porphyry'  resembling  in  some  respects  the  Solvsberg 
type;  and  a  glaucophane-bearing  rock  of  somewhat  similar 
characters  is  found  at  Cape  Ann,  Mass.4  These  rocks  have 
not,  however,  the  trachytic  structure,  and  further  differ  in  the 
nature  of  their  ferro-magnesian  component. 

The  most  usual  type  of  orthoclase-porphyry  (orthophyre  of 
Rosenbusch)  is  exemplified  by  dykes  and  sills  in  the  Carboni- 
ferous of  Thuringia,  in  the  Vosges,  and  in  other  districts. 
Besides  the  orthoclase  phenocrysts  there  may  be  some  of 
plagioclase.  The  ferro-magnesian  minerals  are  only  sparingly 
represented,  and  may  be  biotite,  hornblende,  or  augite.  The 
ground-mass  is  holocrystalline  with  the  structure  styled  ortho- 

1  Bull.  Mus.  Comp.  ZooL  Harv.  (1893)  xvi,  227-230. 

2  Eakle,  A.  J.  S.  (1898)  vi,  489-492  ;  Washington,  Journ.  Geol.  (1899) 
vii,  119,  120. 

3  17th  Ann.  Rep.  U.  S.  Geol.  Sur.  (1896)  665,  666,  pi.  XLIII. 

4  Washington,  A.  J.  S.  (1898)  vi,  176-179.     See  also  Journ.  Geol. 
(1899)  vii,  114-119. 


PORPHYRIES:    BOSTONITES.  123 

phyric,    in  which    short    prisms    of    untwinned   felspar  are 
associated  with  some  interstitial  quartz. 

Of  a  different  type  are  the  rocks  forming  North  Berwick 
Law  and  the  Bass  Rock.  They  have  been  described1  under 
the  name  trachyte,  with  the  associated  lavas  which  they 
closely  resemble  (see  Chap.  XII.),  but  may  be  mentioned 
in  this  place.  The  felspar  of  which  they  are  almost  wholly 
composed  is  rich  in  soda  as  well  as  potash,  and  the  non- 
porphyritic,  trachytoid  structure  of  the  rocks  allies  them  with 
the  bostonites.  An  albite-porphyry  has  been  recorded  from 
Ben  Brachaid  in  Sutherland,  containing  albite  phenocrysts  in 
a  ground-mass  essentially  of  the  same  mineral2. 

The  typical  bostonites  occur  at  Marblehead  Neck  near 
Boston,  Mass.3,  in  the  Adirondacks4,  near  Montreal,  at  Liver- 
more  Falls  and  Shackford,  N.H.,  in  the  Apache  Mts,  Tex., 
etc.,  as  dykes  in  connection  with  nepheline-syenite  or  other 
plutonic  rocks,  and  especially  in  intimate  association  with 
dykes  of  lamprophyre  (camptonite).  The  bostonites  consist 
essentially  of  felspar,  quartz  being  never  abundant  and  the 
ferro-magnesian  silicates  typically  absent.  Phenocrysts  may 
or  may  not  be  developed,  the  bulk  of  the  rock  being  a  ground- 
mass  of  little  felspar  rods,  often  with  partial  flow-disposition 
and  recalling  the  structure  of  the  trachytes  (fig.  28,  A).  In 
many  examples  a  high  percentage  of  soda,  with  little  or  no 
plagioclase  evident,  points  to  a  soda-orthoclase  or  anortho- 
clase,  and  indicates  an  affinity  with  the  ceratophyres.  Rocks 
approaching  bostonite  in  character  occur  in  the  Limerick 
district5  and  in  the  western  part  of  Sutherland ;  and  an  allied 
rock  is  described  by  Dr  Flett6  as  forming  a  dyke  at  Onston 
Ness  in  the  Orkneys.  This  rock  carries  felspar  phenocrysts : 

1  Hatch,  Trans.  Eoy.  Soc.  Edin.  (1892)  xxxvii,  123,  124,  pi.  i,  figs. 
3,  4. 

2  Reddle,  M.  M.  (1884)  v,  141. 

3  Wadsworth,  Proc.  Bost.  Soc.  Nat.  Hist.  (1881)  xxi,  290 ;  Sears,  Bull. 
Mus.  Comp.  Zool.  (1890)  xvi,  169-171 ;  Washington,  Joimi.  Geol.  (1899) 
vii,  293. 

4  Kemp  and  Marsters,  Tram.  N.  Y.  Acad.  Sci.  (1891)  xi,  14-16  ;  Bull. 
No.  107  U.  S.  Geol.  Sur.  (1893)  18-22. 

5  Watts,  Guide,  93. 

6  Tr.  Eoy.  Soc.  Edin.  (1900)  xxxix,  872,  873,  pi.  i,  tigs.  1,  2. 


124  RHOMB-PORPHYRIES. 

the  ground-mass  has  the  trachytic  structure  towards  the 
margins  of  the  dykes,  but  is  allotriomorphic  in  the  central 
part. 


B 


FIG.  28. 

A.  Bostonite,  Marblehead  Neck,  Massachusetts ;   consisting  essen- 
tially of  little  crystals  of  felspar  (anorthoclase)  with  fluxional  arrangement 
[2464]. 

B.  Mica-porphyrite,   Colven,    near   Dalbeattie,    Kirkcudbrightshire; 
with  phenocrysts  of  zoned  plagioclase  and  decaying  biotite  [2594]. 

Among  the  Devonian  intrusions  of  the  Christiania  district 
occur    the    singular  rocks    known   as   rhomb-porphyry  -(Ger. 


llhombenporphyr),  and  they  may  be  studied  in  numerous 
boulders  in  Holderness  and  the  Eastern  Counties.  The  pheno- 
crysts of  potash-soda-felspar,  with  their  unusual  crystallo- 
graphic  development,  have  been  alluded  to  above.  The 
crystals  are  often  rounded  and  corroded,  and  they  contain 
numerous  inclusions  of  materials  like  the  ground-mass.  Some 
of  the  rocks  contain  pseudomorphs  after  olivine.  The  fine- 
textured  holocrystalline  ground-mass  consists  of  short  prisms 
of  felspar  (probably  orthoclase)  with  little  granules  of  augite. 
Apatite  is  often  plentiful,  and  grains  of  titaniferous  iron-ore 
occur.  Rhomb-porphyries  have  been  discovered  by  Osann 
in  the  Apache  Mts,  Tex. 


CERATOPHYRES  :    TINGUAITES.  125 

The  name  ceratophyre  was  first  used  by  von  Giimbel  for 
a  rather  varied  group  of  rocks  in  the  Fichtelgebirge.  Some- 
what similar  rocks  have  been  described  from  Saxony,  West- 
phalia, the  Harz,  and  other  areas.  Porphyritic  quartz  does 
not  occur  in  the  ceratophyres  proper,  and  felspar  is  the 
predominant  mineral  in  both  phenocrysts  and  ground-mass. 
The  phenocrysts  have  the  peculiarities  attributed  to  anor- 
thoclase  or  to  a  cryptoperthite  intergrowth.  The  commonest 
ferro-magnesian  element  is  a  pale  augite  (diopside).  The 
felspar  prisms  of  the  ground-mass  may  be  short  and  unstriated 
or  lath-shaped  and  striated,  and  the  more  acid  members  have 
a  little  interstitial  quartz. 

Rosenbusch  has  given  the  name  tinguaite  to  certain  '  dyke- 
rocks '  which  have  the  composition  of  the  (plutonic)  nephe- 
line-syenites  and  the  (volcanic)  phonolites,  with  structural 
characters  which  place  them  between  those  two  families.  Such 
rocks  are  associated  with  nepheline-syenites  in  the  Serra  do 
Tingua  and  other  places  in  Brazil,  and  in  Massachusetts 
(Essex  County),  Arkansas'  (Fourche  Mt),  and  Texas  (Apache 
Mts).  Phenocrysts  of  orthoclase,  often  with  marked  tabular 
habit  and  with  the  characters  of  sanidine,  are  embedded  in 
a  fine-textured  holocrystalline  ground-mass  of  orthoclase  with 
nepheline,  icgirine,  etc.  This  ground  is  typically  allotrio- 
morphic:  when  the  little  felspars  take  on  the  lath-shape 
with  fluxional  arrangement,  the  rocks  do  not  differ  essentially 
from  phonolites.  There  may, be  phenocrysts  of  nepheline, 
and  in  one  type  (leucite-tinguaite)  large  pseudomorphs  of 
orthoclase  and  elseolite  occur  in  the  form  of  leucite.  This 
latter  type  has  been  described  from  Brazil2,  Arkansas3  (Magnet 
Cove),  and  Montana4.  From  the  last-named  state  comes  also 
a  variety  intermediate  between  the  true  tinguaite  and  the 
Solvsberg  type  mentioned  above  (Landusky  in  the  Little 
Rocky  Mountains5).  A  more  basic  nepheline-bearing  type, 


1  J.  F.  Williams,  Igneous  Rocks  of  Arkansas,  vol.  ii  of  Ann.  Rep.  Geol. 
Sur.  Ark.  for  1890,  100-106. 

2  Derby,  Q.  J.  G.  S.  (1891)  xlvii,  251-265. 

3  J.  F.  Williams,  I.e.  277-286. 

4  Pirsson,  A.  J.  S.  (1895)  1,  394-398. 

5  Weed  and  Pirsson,  Journ.  of  Geol.  (1896)  iv,  419-421. 


126  DIORITE-PORPHYRITES,  ETC. 

on  the  other  hand,  occurs  at  Magnet  Cove,  Ark.1,  and  at 
Beemerville,  NJ.2,  having  phenocrysts  of  nepheline  up  to  an 
inch  in  diameter  in  a  tinguaitic  ground-mass  composed  chiefly 
of  nepheline,  charged  with  segirine-needles,  with  some  ortho- 
clase,  etc.  A  tinguaitic  rock  at  Pickard's  Point,  Mass.3, 
contains  analcime  and  nepheline  as  the  main  elements  of  its 
ground-mass,  and  this  analcime  is  considered  to  be  a  primary 
mineral4.  Here  may  be  mentioned  also  a  remarkable  dyke- 
rock  (Heron  Bay  type)  from  the  Lake  Superior  region, 
consisting  to  the  extent  of  about  one-half  of  analcime,  in 
which  are  embedded  orthoclase,  labradorite,  and  segirine5. 

Coming  now  to  rocks  of  dioritic  affinities,  we  may  mention 
a  qu&rtz-diorite-porpkyrite,  from  Sweet  Grass  Hills,  Montana6, 
and  a  quartz-mica-diorite-pprphyrite,  approaching  granite- 
porphyry,  from  Electric  Peak  in  the  Yellowstone  Park7.  This 
has  abundant  small  phenocrysts  of  felspars,  quartz,  and  biotite, 
with  a  little  hornblende,  and  a  granular  ground-mass  of  felspar 
and  quartz.  In  the  same  district  occur  porphyrites,  generally 
kornblende-porphyrites8,  carrying  abundant  phenocrysts  of  lime- 
soda-felspar  and  hornblende,  with  usually  biotite  and  oc- 
casionally uralitized  augite,  in  a  fine-grained  ground-mass. 
When  the  latter  is  rich  in  quartz,  this  mineral  tends  to  form 
micropoecilitic  patches  enclosing  the  little  felspar-prisms ;  when 
quartz  is  scarce,  the  felspars,  which  are,  at  least  in  the  main, 
plagioclase,  tend  to  have  a  felted  arrangement.  The  ground- 
mass  also  contains  some  hornblende  and  biotite.  Resembling 
the  Electric  Peak  rocks,  and  like  them  of  somewhat  acid 
character  as  a  whole,  are  the  hornblende-porphyrites  and 
hornblende-mica-porphyrites  described  by  Cross9  from  the 

1  J.  F.  Williams,  I.e.  259-261. 

2  Kemp,  Trans.  N.  Y.  Acad.  Sci.  (1892)  xi,  66,  67.     This  type  is  the 
'  sussexite '  of  Brogger,  constituting  the  most  basic  member  of  a  '  rock- 
series  '   of  which  the  other  members  are  grorudite,  solvsbergite,  and 
tinguaite. 

3  Sears,  Bull.  Essex  Inst.  (1893)  xxv. 

4  Washington,  A.  J.  S.  (1898)  vi,  182-186. 

5  Coleman,  Rep.  Bur.  Mines  Toronto  (1899)  viii,  part  2,  172,  173. 

6  Weed  and  Pirsson,  A.  J.  S.  (1895)  1,  311. 

7  Iddings,  12th  Ann.  Rep.  U.  S.  Geol.  Sur.  (1892)  617,  618. 

8  Ibid.  588-594. 

9  Cross,  Uth  Ann.  Rep.  U.  S.  Geol.  Sur.  (1891). 


MICA-PORPHYRITES.  127 

laccolites  and  associated  intrusions  of  the  Henry  and  Abajo 
Mts  in  Utah,  the  West  Elk  and  El  Late  Mts  in  Colorado,  etc. 
Among  the  phenocrysts  the  dominant  minerals  after  plagioclase 
felspar  (oligoclase  or  andesine)  are  hornblende  and  to  a  less 
extent  biotite,  while  augite  and  hypersthene  occur  only  locally. 
Quartz  is  also  developed  porphyritically  and  in  certain  cases 
large  crystals  of  orthoclase,  which,  however,  seem  to  belong 
rather  to  the  same  stage  of  consolidation  as  the  ground-mass 
(Mt  Carbon  and  Gothic  Mt,  in  the  West  Elk  group,  etc.). 
The  ground-mass  is  essentially  an  aggregate  of  orthoclase  and 
quartz,  llocks  generally  resembling  the  above  are  described 
by  Pirsson1  from  the  Judith  Mts  in  Montana.  There  are 
transitional  varieties  between  diorite-porphyrite  and  syenite- 
porphyry  or  quartz-syenite-porphyry. 

Various  rocks  of  the  porphyrite  family  are  known  in  the 
British  Isles,  and  especially  in  Scotland.  Numerous  mica- 
porphyrite  dykes,  of  Old  Red  Sandstone  age,  occur  in  the 
Cheviots2.  The  felspar  phenocrysts  (oligoclase-andesine)  are 
frequently  rounded,  and  shew  carlsbad  and  albite  twinning. 
The  biotite-flakes  are  often  bent,  and  sometimes  shew  a 
resorption  border.  A  colourless  augite  may  also  occur,  and 
magnetite  and  apatite  are  minor  constituents.  The  ground- 
mass  is  microcrystalline,  fine-textured,  and  often  obscured 
by  decomposition.  Quartz  plays  a  variable  part  in  it,  and 
there  are  some  transitions  to  granophyre  and  quartz-porphyry. 
Indeed  the  mica-porphyrites  in  general  often  carry  a  notable 
amount  of  quartz  in  their  ground-mass.  The  handsome  rock 
which  forms  large  intrusive  sills  in  the  Torridon  Sandstone  of 
Cansip,  Sutherland,  may  also  be  placed  here.  It  has  large, 
frequently  broken,  phenocrysts  of  albite-oligoclase,  and  ortho- 
clase also  occurs,  sometimes  intergrown  with  the  plagioclase. 
The  dominant  coloured  mineral  is  biotite,  but  Mr  Teall  also 
notes  augite,  either  colourless  or  green  or  the  former  bordered 
by  the  latter.  Calcite  pseudomorphs  in  the  form  of  augite 
are  common.  These  minerals,  with  some  magnetite,  are  set  in 
a  fine  microcrystalline  ground-mass  of  felspar  and  quartz. 

1  18th  Ann.  Rep.  U.  S.  Geol.  Sur.  (1898)  part  in,  562-564. 

2  Watts,  Mem.  Geol.  Sur.  Eng.  and  Wales,  Expl  Sh.  110  S.W.  (1895) 
62,  63 ;  Kynaston,  Tr.  Edin.  G.  ,9.  (1899)  vii.  398-402. 


128  HORNBLENDE-PORPHYRITES. 

The  dykes  described  by  Mr  Teall1  in  Kirkcudbrightshire 
are  mostly,  mica-hornblende-porphyrites.  The  phenocrysts  are 
of  zoned  plagioclase  in  large  individuals,  green  hornblende 
and  brown  biotite,  both  in  good  crystals,  and  sometimes 
corroded  grains  of  quartz,  while  the  fine-textured  ground-mass 
contains  quartz  and  orthoclase  in  addition  to  the  other 
minerals  named.  In  some  varietes  the  hornblende  is  almost 
or  quite  wanting  (fig.  28,  B). 

Of  hornblende-porphyrites  we  may  recognize  more  than 
one  variety.  Some  of  the  Scottish  examples  are  of  tonalitic 
rather  than  dioritic  affinities  (Cowal  district  of  Argyllshire2). 
Again,  there  are  the  rocks  which  form  sills  of  Lower  Palaeozoic 
age  in  the  Assynt  district  of  Sutherland  (Inchnadampf,  etc.3}. 
Here  the  hornblende  is  green  and  in  very  perfect  crystals, 
often  twinned  :  they  sometimes  shew  zonary  colouring,  and 
are  occasionally  hollow.  A  colourless  augite  in  imperfect 
crystals  sometimes  accompanies  the  hornblende.  The  plagio- 
clase phenocrysts  shew  strong  zonary  banding  between  crossed 
nicols.  Magnetite  and  apatite  are  present  sparingly.  The 
microcrystalline  ground-mass  is  of  felspar  with  subordinate 
quartz.  These  rocks  are  part  of  a  variable  set  of  intrusions. 
On  the  one  hand  is  a  non-porphyritic  and  coarser-textured 
type  with  allotriornorphic  felspar  (diorite),  on  the  other  a 
type  with  more  abundant  hornblende  in  two  generations 
and  with  a  panidiomorphic  ground-mass  (camptonite,  see 
Chap.  X.  and  fig.  33). 

A  hornblende-porphyrite  of  basic  composition  is  seen  in 
the  Mawddach  valley,  near  Dolgelly.  It  contains  large  and 
rather  irregularly  bounded  twin-crystals  of  brown  hornblende 
in  a  much  decomposed  matrix.  Mr  Phillips4  termed  this 
hornblende  uralite,  but*  there  is  no  clear  evidence  that  it  is 
other  than  an  original  mineral. 

The  rocks  to  which  the  name  augite-porphyrite  has  been 
applied  by  German  petrologists  seem  to  be  for  the  most  part 

1  Mem.  Geol.  Sur.  Scot.,  Expl.  Sh.  5  (1896),  44,  45,  and  Silur.  Rocks 
Scot.  (1899)  626,  627. 

2  Teall,  Mem.  Geol.  Sur.  Scot.,  Geol.  of  Cowal  (1897),  103. 

3  Teall,  G.  M.  1886,  346-350. 

4  Q.  J.  G.  S.  (1877)  xxxiii,  427-429,  pi.  xix. 


AUGITE-PORPHYKITES.  129 

old  augitic  lavas,  though  intrusive  types  are  also  included. 
Such  rocks,  probably  of  Triassic  age,  are  represented  in  the 
Monzoni  district  in  the  southern  Tirol.  Augite  is,  however, 
a  frequent  accessory  mineral  in  the  hornblende-porphyrites, 
and  in  particular  occurrences  may  become  the  dominant 
coloured  element  of  the  rock.  Thus  in  the  Henry  Mts  Cross 
remarks  augite-porphyrites  at  Mount  Pennell  and  Mount 
Killers ;  but  these  are  mainly  from  sheets,  while  the  great 
laccolites  themselves  are  of  the  hornblendic  type. 


H.  P. 


CHAPTER  IX. 


DIABASES. 

THE  larger  intrusive  bodies  of  hypabyssal  pyroxenic  rocks, 
whether  intermediate  or  basic  in  composition,  have  petro- 
graphical  features  which  characterize  them  as  a  group  with 
considerable  individuality.  It  is  to  these  rocks  that  we  shall 
apply  the  name  diabase.  Like  their  plutonic  equivalents,  the 
gabbros,  they  are  holocrystalline  and  typically  non-porphyritic ; 
but  they  differ  from  the  normal  gabbros  in  their  less  coarse 
texture,  in  the  absence  of  diallagic  and  other  '  schiller '  struc- 
tures, and  in  the  mutual  relations  of  the  felspar  and  augite 
which  are  'their  two  chief  constituents.  In  these  respects 
there  are,  however,  transitions  between  the  two  sets  of 
rocks. 

The  diabases  occur  as  large  dykes,  sills,  and  laccolitic 
or  other  masses.  Smaller  intrusions  of  rocks  having  a  similar 
chemical  composition  commonly  have  more  of  the  petro- 
graphical  characters  of  volcanic  rocks.  For  these  we  shall 
retain  the  names  dolerite,  andesite,  basalt,  etc.,  and  they  will 
be  excluded  from  this  place. 

The  name  diabase  has  been,  and  still  is,  employed  in 
different  senses.  By  the  German  school  it  is  restricted  to 
the  older  rocks,  whether  hypabyssal  or  volcanic,  dolerite  and 
basalt  being  terms  reserved  for  rocks  of  Tertiary  or  later  age. 
Mr  Airport  shewed  very  conclusively  that  such  a  distinction 
corresponds  with  no  real  difference  between  the  older  and  the 
newer  rocks,  and  he  abandoned  the  name  diabase  in  favour 


FELSPARS  OF  DIABASES.  131 

of  dolerite  for  all.  The  rocks  so  designated  by  Allport  include 
some  of  the  hypabyssal  and  others  of  the  volcanic  type. 
English  writers  have  followed  him  in  admitting  no  criterion 
of  geological  age  into  their  classification  and  nomenclature, 
but  some  of  them  have  inconveniently  employed  the  name 
diabase  for  a  more  or  less  decomposed  dolerite. 

According  to  the  absence  or  presence  of  the  basic  silicate 
oli vine,  the  rocks  of  the  present  family  are  often  divided  into 
diabases  proper  and  olivine -diabases.  Olivine  is  in  general 
found  in  the  more  basic  members  of  the  family,  but  this 
division  does  not  correspond  very  exactly  with  the  chemical 
division  into  intermediate  (or  sub-basic)  and  basic.  By  the 
presence  of  some  other  special  mineral  we  may  distinguish 
such  types  as  quartz-diabase,  bronzite-diabase,  and  h&rnblende- 
diabase;  or  again  quartz-bronzite-diabase  and  olivine-horn- 
blen  de-diabase. 

Various  other  names  have  been  used  for  particular  types 
of  diabasic  rocks.  Among  the  hornblende-bearing  diabases 
of  the  Fichtelgebirge  von  Giimbel  distinguished  two  types ; 
proterobase,  containing  original  hornblende  in  addition  to 
augite,  and  epidiorite,  in  which  the  hornblende  is  all  derived 
from  augite.  Some  writers  have  extended  these  names  to 
cover  all  diabasic  rocks  characterized  by  primary  and  secondary 
hornblende  respectively1.  The  old  field-term  'greenstone,' 
referring  to  the  staining  of  the  rocks  by  chloritic  and  other 
decomposition-products,  included  not  only  diabases  but  diorites, 
picrites,  altered  dolerites,  etc.,  and  so  had  no  precise  significa- 
tion. 

Constituent  Minerals.  The  felspars  of  the  diabases 
range  from  oligoclase  to  anorthite  in  different  examples : 
varieties  of  labradorite  are  perhaps  the  most  common.  The 
crystals  have  a  strong  tendency  to  idiomorphism,  with  colum- 
nar or  sometimes  tabular  habit.  Twin-lamellation  on  the 
albite  law  is  universal,  and  is  often .  combined  with  carlsbad 
twinning,  but  the  pericline  law  is  not  so  common.  Zonary 
growth  is  not  often  shewn,  except  when  a  later  set  of 

1  It  is  probable,  however,  that  secondary  hornblende  has  often  been 
mistaken  for  primary. 

9—2 


132  PYROXENES   OF   DIABASES. 

felspars  occurs,  of  shapeless  outline  and  more  acid  composition  ; 
these  shew  strong  zoning  between  crossed  nicols.  Inclusions 
are  not  common,  except  glass-cavities  and  needles  of  apatite. 
Decomposition  gives  rise  to  calcite-dust,  to  finely  divided 
material,  which  maybe  mica,  to  zeolites,  or  to  granular  epidote. 
The  crystals  also  become  charged  with  strings  and  patches 
of  green  chloritic  substances,  probably  derived  in  part  from  the 
pyroxene. 

The  common  pyroxenic  constituent  is  an  augite,  usually 
without  crystal  outlines.  It  varies  in  thin  slices  from  brown 
to  nearly  colourless,  and  rarely  shews  sensible  pleochroism. 
Zonary  and  '  hour-glass '  structures  are  sometimes  seen.  The 
orthopinacoidal  twin  is  common,  and  in  some  cases  there  is  a 
fine  basal  lamination1  in  addition  (Whin  Sill).  The  common- 
est decomposition-products  are  pale  green,  fibrous  or  scaly 
aggregates  of  serpentinous  and  chloritic  substances.  The 
former  may  be  recognized  by  their  low  refractive  index  and 
moderately  high  birefringence ;  the  latter  are  usually  very 
feebly  birefringent  or  sensibly  isotropic,  and  shew  distinct 
pleochroism.  Another  change  to  which  augite  is  subject  is 
that  which  results  in  a  light-green  'uralitic'  hornblende. 
This  is  usually,  but  not  always,  fibrous  in  structure. 

Some  diabases  contain  bronzite  in  addition  to  augite.  It 
is  in  more  or  less  idiomorphic  crystals,  with  faint  pleochroism, 
and  gives  rise  by  alteration  to  pseudomorphs  of  light  green 
fibrous  bastite.  Only  occasionally  does  hornblende  appear 
as  an  original  constituent.  It  seems  to  be  characteristically 
a  brown  variety.  Brown  biotite  is  also  a  rare  accessory. 

A  little  quartz  is  found  in  some  of  the  less  basic  diabases, 
occurring  interstitially.  Whether  it  is  original  or  a  decom- 
position-product is  sometimes  difficult  to  decide,  but  when  the 
mineral  forms  part  of  a  micrographic  intergrowth  with  felspar 
its  primary  nature  may  safely  be  assumed. 

The  olivine,  which  occurs  in  very  many  diabases,  builds 
more  or  less  rounded  idiomorphic  crystals  or  grains,  sensibly 
colourless  or  very  pale.  It  has  the  same  mode  of  alteration 
as  in  the  olivine-gabbros  and  peridotites. 

1  Teall,  Q.  J.  G.  S.  (1884)  xl,  pi.  xxix,  fig.  1. 


IRON-ORE  MINERALS   OF   DIABASES. 


133 


The  iron-ores,  which,  in  contrast  with  some  gabbros,  the 
diabases  contain  abundantly,  include  ilmenite  and  magnetite. 
The  two  are  very  commonly  associated,  and  some  so-called 
titaniferous  magnetite  has  been  supposed  to  be  a  minute  inter- 
growth  of  the  two1.  They  are  easily  distinguished  when  they 
occur  as  crystals  or  skeleton-crystals.  In  most  cases  the 
ilmenite  has  given  rise  to  more  or  less  of  its  characteristic 


FIG,  29.    DECOMPOSING  DIABASE,  DENEIO,  NEAR  PWLLHELI, 
CAERNARVONSHIRE  ;    x  20. 

This  shews  decomposing  felspar-crystals  and  ophitic  augite,  with 
ilmenite -skeletons  (il),  crusted  with  leucoxene,  and  patches  of  radiating 
fibres  of  a  zeolitic  mineral  (z)  [123]. 

decomposition-product,   grey   cloudy  masses    of    semiopaque 
leucoxene2  (fig.  29). 

Long  columnar  or  needle-like  crystals  of  apatite  occur  in 
most  diabases,  but  in  some  are  capriciously  distributed. 

Structure.  As  regards  structure,  the  diabases  offer  a 
contrast  to  normal  plu tonic  rocks,  owing  mainly  to  the  fact 
that  the  crystallization  of  the  felspar  has  preceded  that  of  the 

1  Teall,  Q.  J.  G.  S.  (1884)  xl,  650,  651,  pi.  xxix,  figs.  4-7. 
3  Cohen  (3),  pi.  LXI,  fig.  4  ;  Teall,  pi.  xvn,  fig,  2. 


134  STRUCTURES   OF   DIABASES. 

dominant  ferro-magnesian  constituent.  As  seen  in  a.  slice, 
the  columnar  crystals  of  felspar  -shew  more  or  less  elongated 
sections,  with  no  law  of  arrangement,  and  around  or  between 
these  the  augite  is  moulded.  The  last-named  mineral  in  most 
cases  distinctly  wraps  round  the  felspar  crystals,  and  often 
forms  plates  of  some  extent,  enclosing  many  of  them.  This  is 
known  as  the  ophitic  structure1  (fig.  30).  In  other  cases  the 
augite  tends  to  form  more  or  less  rounded  grains  embedded  in 
a  plexus  of  lath-shaped  felspars,  adjacent  grains  not  being 
parts  of  one  crystal  but  shewing  different  orientations.  This 
is  what  Prof.  Judd2  has  styled  the  granulitic  structure  :  he 
considers  it  due  to  movement  towards  the  end  of  the  process 
of  consolidation.  In  both  types,  if  olivine  is  present  it  is 
always  idiomorphic  towards  the  augite,  but  may  be  penetrated 
by  the  felspar  prisms.  The  rhombic  pyroxene,  too,  is  con- 
stantly of  earlier  crystallization  than  the  augite,  and  may 
shew  good  outlines.  The  iron-ores  are  often  idiomorphic, 
but  magnetite  may  be  in  part  later  than  the  felspar.  When, 
as  is  sometimes  the  case,  a  subordinate  felspar,  of  later  con- 
solidation than  the  dominant  kind,  is  present,  it  has  crystallized 
with  or  after  the  augite,  and  is  always  shapeless. 

The  typical  diabases  thus  present  a  very  uniform  structural 
character,  which  in  its  best  development  is  almost  peculiar  to 
them.  In  a  few  diabases,  however,  the  augite,  especially  if 
not  abundant,  is  partially  idiomorphic,  and  the  same  is  true  of 
rocks  which  are  on  the  border-line  between  diabase  and 
gabbro.  A  porphyritic  character,  due  to  the  development  of 
relatively  large  crystals  of  felspar  at  an  early  stage,  is  not 
common  :  it  is  sometimes  connected  with  an  increasing  fine- 
ness of  texture  of  the  rock  on  approaching  the  edge  of  an 
intrusive  mass.  Other  occasional  marginal  peculiarities  are 
flow-phenomena,  vesicles  or  amygdules,  and  the  development  of 
a  glassy  base  or  sometimes  of  variolitic  and  allied  structures. 
Rocks  having  these  features  and  occurring  as  marginal  modifi- 
cations of  normal  diabases  do  not  differ  in  any  essential  from 
certain  types  of  lavas,  and  will  therefore  not  be  noticed  in 
this  place. 

1  See  chromolithograph  of  diabase  in  Berwerth,  Lief.  1. 
a  Q.  J.  G.  S.  (1886)  xlii,  pp.  68,  76,  and  figs.  pi.  v. 


QUARTZ-DIABASES.  135 

Leading  types.  A  true  quartz-diabase  is  not  often  met 
with.  In  any  but  quite  fresh  rocks,  at  least,  it  is  not  possible 
to  be  certain  that  quartz  occurring  interstitially  is  really  an 
original  constituent  of  igneous  origin.  Among  the  numerous 
dykes  traversing  the  old  gneiss  of  Sutherland  are  diabases  of 
which  some  are  quartz-bearing  (Loch  Glencoul,  etc.).  The 
chief  constituent  minerals  are  a  basic  plagioclase  and  a  pale 
or  colourless  augite,  the  relations  between  the  two  being 
rather  variable.  A  green  or  yellow-green  hornblende  occurs 
as  a  marginal  alteration  of  the  augite,  especially  around  the 
grains  of  magnetite,  and  a  little  brown  biotite  is  also  associated 
with  the  latter.  The  hornblende  is  connected  with  mechanical 
stress  in  the  rock,  and  specimens  may  be  collected  to  shew  the 
complete  amphibolization  of  the  augite. 

A  well-known  rock  in  the  north  of  England  is  the  Great 
Whin  Sill1,  which  is  intrusive  in  Lower  Carboniferous  strata, 
and  extends  from  the  Northumberland  coast  to  the  Eden 
valley.  In  its  coarser  central  parts  it  sometimes  approaches  a 
gabbro  in  aspect,  the  augite  becoming  idiomorphic ;  the  fine- 
textured  portions  near  the  margin,  on  the  other  hand,  take  on 
an  andesitic  character,  developing  perhaps  some  glassy  base ; 
but  the  bulk  of  the  intrusion  is  of  diabase  of  a  distinctive 
type.  The  normal  structure  is  more  or  less  ophitic,  and  the 
dominant  constituents  are  a  lath-shaped  felspar,  near  andesiue, 
and  a  pale  brown  augite,  often  with  basal  striation.  The 
iron-ore  is  titaniferous,  and  may  perhaps  represent  minute 
intergrowths  of  magnetite  and  ilmenite.  An  accessory  mineral 
is  bronzite,  tending  to  be  replaced  in  the  usual  fashion  ;  brown 
mica  is  occasionally  seen,  and  a  little  brown  hornblende  is 
often  present,  bordering  the  augite  with  crystallographic  re- 
lation. Quartz  is  detected  in  all  the  coarser  varieties  of  the 
rock,  and  is  at  least  in  part  original,  since  it  frequently  occurs 
in  micrographic  intergrowth  with  felspar.  The  rock  is  thus  a 
quartz-diabase.  Mr  Teall3  has  described  a  similar  rock  from 
Ratho,  near  Edinburgh.  With  these  rocks  we  may  also  com- 
pare that  near  Stirling3.  The  general  mass  of  this  is  a  simple 
diabase,  the  augite  often  shewing  basal  striation,  but  there 

1  Teall,  Q.  J.  G.  S.  (1884)  xl,  640-657,  pi.  xxix  :   also  Brit.  Petr. 
pi.  xin.  tig.  2. 

2  Teall,  p.  190.  »  Monckton,  Q.  J.  G.  S.  (1895)  li,  480-491. 


136  QUARTZ -DIABASES,   ETC. 

are  coarse-textured  veins,  which  contain  quartz  in  delicate 
micrographic  intergrowth  with  part  of  the  felspar. 

The  Penmaenmawr1  intrusion,  probably  of  Ordovician  age, 
is  also  characterized  by  quartz  occurring  interstitially  in  a 
micrographic  intergrowth.  In  this  rock  bronzite  becomes  an 
essential  constituent,  being  quite  as  abundant  as  the  pale 
brown  augite.  The  latter  mineral  often  shews  the  delicate 
basal  striation  already  noticed.  Biotite  is  sometimes  rather 
abundant,  but  the  dominant  type  of  rock  is  a  quartz-bronzite- 
diabase.  The  structure  of  the  rock  is  rather  granulitic  than 
ophitic,  and  it  usually  shews  some  approach  to  the  characters 
of  volcanic  rocks  in  the  occurrence  of  more  than  one  generation 
of  felspar.  Some  of  the  latest  shapeless  crystals  are  to  be 
referred  to  orthoclase.  The  rock  passes  into  a  type  which 
would  be  properly  described  as  an  andesite.  The  general 
body  of  the  rock  is  traversed  by  comparatively  coarse  segrega- 
tion-veins of  more  acid  composition2. 

Quartz-diabases  are  not  unknown  in  America ;  e.g.,  at 
Newhaven3  and  Medford,  Conn.,  in  the  Province  of  Quebec, 
and  near  St  John,  N.B.4 

The  numerous  sills  of  Ordovician  age  in  Caernarvonshire5 
are  of  diabase  without  olivine,  and  have  almost  universally  the 
ophitic  structure.  The  felspar  gives  lath- shaped  or  rectangular 
sections  from  '05  to  '5  inch  long,  with  albite-  but  only  occa- 
sionally pericline-lamellation :  it  often  gives  extinction-angles 
indicating  labradorite  and  neighbouring  varieties.  The  augite 
is  pale  brown  to  almost  colourless,  and  very  rarely  shews  any 
approach  to  idiomorphism.  Besides  the  commoner  decomposi- 
tion-products, there  is  often  a  fibrous  colourless  hornblende, 
fringing  the  augite  but  occupying  the  place  of  destroyed  felspar, 
etc.  The  iron  ores  include  both  magnetite  and  ilmenite,  often 
together,  and  apatite  is  locally  plentiful.  Rhombic  pyroxene 
is  wanting,  as  well  as  olivine,  while  original  hornblende  and 


1  Bala  Vole.  Ser.  Caern.  (1889)  65  ;  Teall,  pi.  xxxv,  fig.  2. 

2  Waller,  Midland  Naturalist  (1885),  viii,  1-7. 

3  Pirsson,  in  Diller,  264-273. 

4  Matthew,  Trans.  N.  Y.  Acad.  Sci.  (1895)  xiv,  213,  214,  pi.  xv^fig.  2. 

5  Bala  Vole.  Ser.  Caern.  (1889)  75-86. 


BRITISH  DIABASES.  137 

quartz  are  practically  absent,  and  biotite  very  exceptional. 
These  Caernarvonshire  diabases  are  thus  of  very  simple  mine- 
ralogical  constitution.  Despite  the  absence  of  olivine,  they 
are  of  thoroughly  basic  composition.  The  diabases  of  similar 
age  in  Wicklow  are  also  free  from  olivine,  and  are  probably  of 
more  acid  composition,  some  of  them  containing  quartz.  They 
are  characterized  by  a  partial  or  even  total  conversion  of  the 
ophitic  augite  into  hornblende,  with  other  changes  ascribed  to 
dynamic  metamorphism1. 

A  different  type  is  presented  by  the  post-Carboniferous 
(probably  Tertiary)  dykes  found  in  the  northern  half  of 
England  and  in  North  Wales,  e.g.  on  the  Menai  Straits. 
The  smaller  ones  are  augite-andesites ;  the  larger  may  be 
classed,  as  dolerites  or  as  diabases  shewing  a  tendency  to  a 
doleritic  type.  The  dominant  felspars  give  the  usual  rect- 
angular section,  and  the  light  brown  augite  moulds  round 
them  in  ophitic  fashion ;  but  a  special  feature  of  the  rocks  is 
the  occurrence  of  a  second  and  subordinate  generation  of 
felspar  in  allotriomorphic  crystal-grains  which  have  consolid- 
ated, on  the  whole,  about  simultaneously  with  the  augite. 
They  have  less  close  twin-lamellation  than  the  dominant 
felspars,  are  of  more  acid  composition,  and  always  shew  a 
marked  zonary  banding  between  crossed  nicols.  These  rocks 
contain  magnetite,  but  not  ilmenite.  Very  similar  dykes,  of 
Tertiary  age,  are  abundant  in  some  parts  of  Scotland  and  the 
Inner  Hebrides. 

Numerous  olivine-diabases  are  associated  with  the  Car- 
boniferous strata  of  the  Midlands.  Good  examples  are  seen 
in  the  Glee  Hills,  Shropshire2.  The  rock  of  Pouk  Hill3,  near 
Walsall,  is  an  ophitic  olivine-diabase.  In  that  of  Rowley, 
near  Birmingham,  the  augite  occurs  in  little  grains  and  tends 
to  be  idiomorphic4,  or  again  there  is  a  micrographic  inter- 
growth  of  augite  and  felspar5.  In  this  rock  are  relatively  acid 


1  Hatch,  G.  M.  (1889)  263-265. 

2  This  and  many  other  British  examples  were  noticed  by  Allport, 
Q.  J.  G.  S.  (1874)  xxx,  529-567. 

3  Watts,  Pr.  Geol.  As*.  (1898)  xv,  397-400. 

4  Teall,  pi.  xi. 

5  Ibid.  pi.  xxm,  fig.  2, 


138 


BRITISH    OLIVINE-DIABASES. 


segregation-veins,  in  which  part  of  the  felspar  is  orthoclase1. 
A  few  of  the  Derbyshire  '  toad-stones '  have  the  structure  of 
ophitic  diabases2  (tig.  30),  and  in  some  of  them  Mr  Arnold- 
Bemrose3  has  described  certain  peculiar  pseudomorphs  after 
olivine. 


ol 


CM- 


lb 


FIG.  30.     OLIVINE-DIABASE,  BONSALL,  DERBYSHIRE  ;    x  20. 

Shewing  olivine-grains  (ol),  more  or  less  completely  serpentinized, 
magnetite  (mg),  and  lath-shaped  crystals  of  labradorite  (lb),  set  in  a 
framework  of  crystalline  augite  (au),  which  wraps  round  and  encloses 
the  felspar  with  typical  ophitic  structure  [424]. 


Numerous  intrusions  of  oli vine-diabase,  some  of  Carbon- 
iferous and  others  of  Tertiary  age,  occur  in  the  southern  half 
of  Scotland4  and  in  the  western  islands.  As  distinguished 
from  the  basalts  and  dolerites,  they  are  typically  ophitic  rocks 
consisting  of  magnetite,  olivine  (often  in  fresh  crystals),  lath- 
shaped  felspar,  and  crystal-plates  of  augite.  Zeolites  are 

1  Waller,  Midi.  Natst.  (1885)  viii,  261-266. 

2  Teall,  pi.  ix  ;  Arnold-Bemrose,  Q.  J.  G.  S.  (1899)  lv,  pi.  xx,  figs. 
1-3. 

3  Q.  J.  G.  S.  (1895)  li,  613-616,  pi.  xxiv. 

4  The  rock  quarried  at  Corstorphine  near  Edinburgh  is  a  good  example 
of  the  earlier  set :  see  coloured  plate  in  Cole's  Stud,  in  Micro.  Sci.  (1882) 
No.  32.     For  Tertiary  examples  see  Teall,  pi.  x. 


HORNBLENDE-DIABASES:    TESCHENITES.  139 

frequent  among  the  secondary  products.  Sills,  dykes  and 
rocks  of  ophitic  olivine-diabase  are  abundant  also  in  the 
Tertiary  volcanic  plateau  of  Antrim1. 

Without  entering  into  an  account  of  particular  occur- 
rences in  America,  it  may  be  stated  that  dykes  of  diabase, 
and  especially  of  olivine-diabase,  are  widely  distributed  in  the 
Archaean  and  other  ancient  formations  of  Canada  and  the 
northern  United  States2. 

Of  hornblende-bearing  diabases  a  good  example  is  found  in 
a  large  dyke  which  runs  on  the  east  side  of  Holyhead 
Mountain3.  The  brown  hornblende  is  very  frequently  in 
parallel  intergrowth  with  augite,  which  it  tends  to  envelope. 
The  augite  is  a  pale  malacolite  variety.  Apatite  and  magnetite 
are  abundant.  The  structure  of  this  rock  is  very  variable, 
sometimes  the  felspar,  sometimes  the  augite,  presenting  idio- 
morphic  boundaries  to  the  other.  Other  examples  occur  in 
the  neighbourhood  of  Penarfynydd,  near  Sarn,  in  the  south- 
west of  Caernarvonshire.  Of  diabases  containing  derivative 
hornblende  we  have  numerous  examples  in  this  country. 
Many  of  the  '  greenstones '  of  Cornwall  are  much  altered  dia- 
bases shewing  uralitization,  chloritization,  and  other  changes  ; 
but  the  rocks  so  named  in  the  field  include  also  old  basic 
lavas  and  other  types4. 

We  may  briefly  notice  in  this  place  the  peculiar  group  of 
rocks  named  teschenite  by  Hohenegger,  occurring  as  intrusions 
in  the  Cretaceous  of  Silesia  and  Moravia  (Teschen,  Neutitschein, 
Sohla,  etc.\  They  consist  mainly  of  augite,  brown  hornblende, 
plagioclase,  apatite,  and  analcime.  The  augite  is  often  of  a 
violet  tint  and  strongly  pleochroic,  and  it  is  frequently  bor- 
dered by  hornblende  in  parallel  position.  The  apatite  is  very 
abundant  and  builds  large  prisms.  The  analcime  is  in  some  cases 
secondary,  and  has  been  supposed  to  be  derived  from  nepheline, 
while  some  observers  have  recorded  the  presence  of  nepheline 

1  Watts,  Guide,  78. 

2  A  list  of  references  to  described  examples  is  given  by  Kemp   and 
Marsters,  Bull.  No.  107,  U.  S.  Geol.  Sur.  (1893)  28,  29. 

3  G.  M.  1888,  270,  271. 

4  J.  A.  Phillips,  Q.  J.  G.  S.  (1876)   xxxii,  155-178  ;    (1878)   xxxiv 
471-496,  pi.  xx-xxn. 


140  TESCHENITES. 

in  the  rocks.  A  rock  of  teschenitic  affinities  is  found  at  Car 
Craig  in  the  Firth  of  Forth1.  It  is  rich  in  a  purplish  brown, 
pleochroic  augite,  and  contains  altered  felspar,  analcime  and 
other  zeolites,  iron  ores,  and  brown  mica  (probably  secondary). 
It  presents  points  in  common  with  the  neighbouring  picrite  of 
Inchcolm.  All  these  rocks  are  typically  non-ophitic,  but 
others  more  resembling  normal  diabases  have  also  been 
included  under  the  name  teschenite.  An  analcime-bearing 
diabase  of  this  kind  forms  a  massive  sill  at  Dippin  in  the 
south  of  Arran.  It  is  a  coarse-textured  ophitic  rock  composed 
of  olivine,  apatite,  magnetite,  labradorite,  purplish  pleochroic 
augite,  and  clear  interstitial  patches  of  analcime.  There  is 
nothing  to  indicate  that  this  last  mineral  is  of  other  than 
primary  origin.  In  San  Luis  Obispo  County,  California,  Fair- 
banks2 has  described  a  diabase  with  analcime,  which  he  finds 
to  be  in  part  primary. 

1  Teall,  pi.  xxn,  fig.  1. 

2  Ball.  Dep.  Geol.  Univ.  Calif.  (1895)  i,  273-300,  plate. 


CHAPTER  X. 

LAMPROPHYRES. 

THE  lamprophyres  are  a  peculiar  group  of  rocks  occurring 
typically  as  dykes  or  other  small  intrusions.  Chemically  they 
are  characterized  by  containing,  with  a  medium  or  low  silica- 
percentage,  a  considerable  relative  quantity  of  alkalies  (especi- 
ally potash),  while  the  oxides  of  the  diatomic  elements  are  also 
abundantly  represented.  This  shews  itself  in  the  commoner 
types  of  lamprophyres  by  an  abundance  of  brown  mica,  and 
indeed  the  lamprophyres  as  a  family  are  rich  in  ferro-magnesian 
silicates.  They  are  fine-grained  rocks,  but  almost  always 
holocrystalline,  and  their  structure  is  in  some  respects  peculiar. 

Von  Gumbel's  name  lamprophyre  has  been  extended  by 
Rosenbusch  to  coyer  the  various  members  of  this  family.  The 
best  known  varieties  are  mica-la mprophyres  ('mica- traps,'  Ger. 
Glimmertrapp).  Of  these,  two  types  have  long  been  recog- 
nized, a  chief  point  of  distinction  being  the  predominance  of 
orthoclase  in  one  and  plagioclase  in  the  other.  To  these  types 
are  given  the  names,  respectively,  minette  (a  word  taken  from 
the  miners  of  the  Vosges)  and  kersantite  (from  Kersanton, 
near  Brest).  To  these  Kosenbusch  added  two  other  types  for 
rocks  in  which  the  place  of  biotite  is  taken  by  augite  or 
hornblende.  He  separated  those  with  dominant  orthoclase 
(vogesite)  from  those  with  dominant  plagioclase  (camptonite). 
It  should  be  noted  that  the  criterion  of  the  felspars  does  not 
lead  in  this  family  to  a  very  natural  division,  especially  when 
much  of  the  potash  in  the  rocks  is  present  in  mica.  Further, 


142  BIOTITE   OF    LAMPROPHYRES. 

the  decomposition  of  the  rocks  often  renders  the  identification 
of  the  felspars  difficult.  For  most  purposes  it  is  perhaps 
sufficient  to  distinguish  the  rocks  merely  as  mica-,  hornblende-, 
and  augite-lamprophyres.  There  are  other  types  of  very  basic 
composition,  which  are  devoid  of  felspar,  and  these  we  shall 
group  under  the  name  monchiquite. 

The  rocks  of  this  family  have  a  wide  range  of  chemical 
composition.  Their  equivalents,  from  this  point  of  view, 
among  the  volcanic  types  are  chiefly  basaltic  rocks,  and 
especially  leucite-  and  nepheline-bearing  basalts.  From  these 
the  lamprophyres  as  a  whole  differ  considerably  in  mineralo- 
gical  composition,  olivine  being  wanting  or  poorly  represented 
in  many  of  the  types,  and  the  felspathoid  minerals  occurring 
only  very  exceptionally  ;  while,  on  the  other  hand,  brown 
mica,  a  mineral  by  no  means  characteristic  of  basaltic  lavas, 
is  a  prominent  constituent  in  many  of  the  lamprophyres. 

Constituent  minerals.  The  characteristic  mineral  of 
those  lamprophyres  most  usually  met  with  is  biotite,  which 
occurs  in  hexagonal  flakes.  The  extinction-angle  (3°  or  4°)  is 
sufficient  to  shew  frequently  a  lamellar  twinning  parallel  to 
the  basal  cleavage.  The  flakes  are  very  commonly  bleached 
in  the  interior,  retaining  only  at  the  margin  the  normal  deep 
brown  colour  (fig.  31,  A).  With  the  bleaching  there  is  a  certain 
diminution  in  birefringence.  More  rarely  we  find  a  dark 
interior  with  a  pale  border,  or  a  dark  nucleus  and  border  with 
a  pale  intermediate  zone.  Complete  decomposition  results  in 
a  pale,  feebly  polarizing  substance  as  a  pseudomorph.  A 
greenish  chloritic  alteration  is  also  found.  Iron-oxide  separates 
out,  usually  as  limonite,  and  other  minerals  are  produced  as 
little  wedges  or  lenticles  along  the  cleavages  of  the  mica 
(fig.  31,  A).  The  titanic  acid  of  the  mica  separates  out  as 
rutile,  in  fine  needles  arranged  in  three  sets  at  angles  of  60° : 
this  is  well  seen  in  basal  sections.  The  original  inclusions 
of  the  biotite  are  apatite,  and  sometimes  magnetite  and  zircon. 

Short  columnar  crystals  of  nugite  occur  in  many  lampro- 
phyres, shewing  sharp  outlines  with  an  octagonal  cross-section, 
and  sometimes  lamellar  twinning.  When  fresh,  the  mineral 
is  pale  green  or  almost  colourless  in  slices,  but  it  is  readily 
replaced  by  serpentine,  calcite,  chlorite,  etc.,  in  good  pseudo- 


MINERALS    OF    LAMPROPHYRES. 


143 


iriorplis  (fig.  31,  C).    In  other  cases  uralitization  may  be  noticed. 
The  augite  crystals  are  sometimes  coated  with  Hakes  of  biotite. 


8 


FIG.  31.     MICA-LAMPROPHYRES  ; 


A.  Helm  Gill,  near  Dent,  Yorkshire.     The  mica-flakes  shew  each  a 
dark  border  and  a  bleached  interior.     There  are  also  lenticles  of  secondary 
products  intercalated  along  the  cleavage-planes  [444]. 

B.  Kawthey  Bridge,  near  Sedbergh,  Yorkshire.     Olivine   has  been 
present  in  abundance,  and  is  now  replaced  by  some  rhombohedral  car- 
bonate with  a  border  of  iron-oxide  [2728]. 

C.  St  Heliers,  Jersey.     Shewing  octagonal  cross-sections  of  augite, 
largely  replaced  by  secondary  products  [1094]. 

The  most  usual  occurrence  of  hornblende  is  in  long  well-shaped 
prisms,  frequently  twinned,  but  it  has  some  variety  of  habit. 
The  colour  is  brown  or  sometimes  green.  The  mineral  may 
be  converted  into  a  chloritic  substance  with  separation  of 
iron-oxides. 

A  striking  feature  in  the  lamprophyres  is  that  the  felspars 
do  not  usually  occur  as  phenocrysts.  The  nature  of  the  felspar 
in  the  more  altered  rocks  can  be  verified  only  after  removing 
the  carbonates  from  the  slice  with  dilute  acid.  The  small 
columnar  or  tabular  crystals  of  plagioclase  shew  albite-lamella- 
tion  and  frequently  zonary  banding.  They  often  have  a  kind 
of  sheaf-like  grouping.  Decomposition,  beginning  in  the 


144  STRUCTURES   OF   LAMPROPHYRES. 

interior,  gives  rise  to  abundant  calcite.  The  orthoclase,  and 
perhaps  anorthoclase,  build  short  rectangular  crystals,  simple 
or  carlsbad  twins,  often  clouded  or  with  ferruginous  staining. 
The  monchiquites  have  no  felspar,  but  some  apparently  contain 
analcime,  always  interstitial. 

Some  of  the  more  acid  lamprophyres  have  a  certain  amount 
of  quartz,  which  is  either  the  latest  product  of  consolidation 
or  is  intergrown  with  a  portion  of  the  felspar  with  micro- 
graphic  structure. 

A  common  accessory  in  some  lamprophyres,  and  an  essential 
in  certain  types,  is  olivine,  which  builds  relatively  large  perfect 
crystals,  or  sometimes  groups  of  rounded  grains.  It  is  occa- 
sionally found  fresh,  but  very  commonly  represented  by 
pseudomorphs  of  carbonates  and  serpentine  (fig.  31,  B). 

The  iron-ores  are  not  often  very  abundant,  and  may  be 
quite  wanting.  The  most  usual  is  pyrites,  but  octahedra  of 
magnetite  are  also  found. 

A  constant  and  abundant  accessory  is  apatite,  but  it  is 
sometimes  in  such  fine  needles  as  to  be  invisible  except  by 
oblique  illumination.  Sphene  and  zircon  are  only  exception- 
ally met  with. 

Structures  and  peculiarities.  Many  of  the  lam- 
prophyres are  non-porphyritic,  with  a  rather  exceptional 
structure  due  to  a  strong  tendency  to  isomorphism  of  all 
the  constituent  minerals.  This  is  the  '  panidiomorphic  '  struc- 
ture of  Rosenbusch1.  The  porphyritic  members  of  the  family, 
again,  have  a  peculiarity,  in  that  the  porphyritic  character  is 
produced  by  a  recurrence  of  the  ferro-magnesian  constituents, 
not  of  the  felspars.  Any  recurrence  of  the  latter,  and  especially 
of  orthoclase,  is  rare,  but  two  generations  of  biotite  or  of  horn- 
blende are  seen  in  many  of  the  rocks.  When  olivine  occurs, 
it  is  in  conspicuous  crystals,  but  only  of  one  generation. 

Without  shewing  any  real  flow-structure,  the  felspars  of 
the  rock  sometimes  have  a  special  grouping  in  sheaf-like  or 
rudely  radiating  fashion.  Exceptionally  orthoclase  is  moulded 

1  See  chromolithograph  of  kersantite  in  Berwerth,  Lief,  i;  and  of 
augite-minette  in  Lief.  in. 


PECULIARITIES    OF    LAMPROPHYRES.  145 

on  the  other  constituents  :  usually  it  is  idiomorphic,  save 
when  it  builds  micrographic  structures  with  quartz.  There 
is  little  indication  of  any  isotropic  residue  in  the  typical 
lamprophyres,  though  in  some  cases  little  ovoid  vesicles,  rilled 
with  secondary  products,  suggest  the  former  presence  of  some 
glassy  matter,  now  perhaps  devitrified.  In  some  of  the 
monchiquites,  however,  there  is  what  has  been  described  as 
a  glassy  base.  The  mica-lamprophyres  are  remarkably  prone 
to  decomposition,  and  often  have  20  or  30  per  cent,  of  calcite 
and  other  secondary  products. 

Grains  of  quartz  and  crystals  of  alkali-felspars  are  found, 
though  very  sparingly,  in  many  lamprophyres.  Their  sporadic 
occurrence  and,  still  more,  some  curious  features  which  they 
invariably  present  compel  us  to  regard  them  as  something 
apart  from  the  normal  constitution  of  the  rock  and  of  quasi- 
foreign  origin.  The  enclnwd  </i((tr/z  (jni/ns  are  of  rounded 

ni 


ni, 

FIG.  32.     OLIGOCLASE  CRYSTAL  ENCLOSED  IN  A  LAMPROPHYRE  DYKE 
AT  GILL  FARM,  NEAR  SHAP  WELLS  ;    x  20. 

Crossed  nicols.  The  crystal  is  rounded  by  magmatic  corrosion  and 
bordered  by  a  narrow  margin  of  orthoclase  (or).  In  addition  to  the 
albite-lamellation  of  the  oligoclase  (a),  there  is  a  carlsbad  twinning  (c) 
common  to  both  felspars  [1155]. 

H.  P.  10 


146  MICA-LAMPROPHYRES. 

form,  with  evident  signs  of  corrosion,  and  are  seen  to  be 
surrounded  by  a  narrow  ring  or  shell  due  to  a  reaction 
between  the  quartz  and  the  surrounding  magma.  This  shell 
is  probably  in  the  first  place  of  augite,  but  it  is  often  found  to 
consist  of  minute  flakes  of  greenish  fibrous  hornblende  or  of 
calcite  and  chloritic  products.  The  quartz  having  this  mode 
of  occurrence  must  be  distinguished  from  genuine  derived 
fragments  torn  from  other  rocks  :  these  are  of  irregular  form, 
often  complex,  and  may  contain  inclusions  unknown  in  the 
corroded  quartz-grains. 

The  enclosed  felspar  crystals  are  always  of  an  acid  species 
—either  orthoclase  or  a  plagioclase  rich  in  soda.  The  crystals 
are  corroded  so  as  to  present  a  rounded  outline,  but  not  re- 
duced to  mere  round  grains.  The  plagioclase  thus  corroded 
is  bordered  by  a  narrow  margin  of  orthoclase  due  to  the  action 
of  the  magma  (fig.  32). 

Illustrative  examples.  The  best-known  British  ex- 
amples occur  as  small  dykes  and  sills  in  the  north  of  England1, 
and  are  of  an  age  between  the  Silurian  and  the  Carboniferous. 
The  dykes  are  numerous  in  the  southern  part  of  the  Lake  Dis- 
trict, from  Windermere  to  Shap  and  on  to  Sedbergh,  and  they 
are  seen  again  in  the  Lower  Palaeozoic  inliers  of  Ingleton,  Eden- 
side,  and  Teesdale.  The  rocks  are  mica-lamprophyres,  but 
many  of  them  contain  subordinate  augite,  always  in  perfect 
crystals,  but  often  decomposed.  The  relative  proportions  of 
orthoclase  and  plagioclase  vary,  so  that  some  examples  would 
be  named  minette  and  others  kersantite,  the  latter  being 
perhaps  the  commoner.  Good  pseudomorphs  after  olivine  are 
seen  in  the  dykes  in  the  Seofoergh  district  (fig.  31,  H).  The 
dykes  at  Cronkley,  in  Teesdale,  have  abundant  pseudomorphs 
with  hexagonal  and  quadrangular  outlines  representing  some 
mineral  not  yet  certainly  identified. 

Scattered  quartz-grains  with  the  characteristic  corrosion- 
border  occur  in  many  of  the  dykes ;  and  felspars,  both  ortho- 
clase and  oligoclase  (fig.  32),  are  enclosed  sporadically  in  the 
Edenside  intrusions,  and  more  abundantly  in  those  to  the  south 
of  the  Shap  granite.  These  rocks  shew  various  transitions  from 
typical  lamprophyres  to  a  micaceous  quartz-porphyry  of  one 

1  G.  M.  1892,  199-206,  with  numerous  references. 


MICA-LAMPROPHYRES.  147 

of  the  less  acid  types,  and  indeed  very  different  kinds  of  rocks 
occur  imperfectly  mingled  in  one  and  the  same  dyke. 

Quartz  does  not  occur  as  a  normal  constituent  in  most  of 
the  north-country  lamprophyres,  though  it  is  found  in  the 
transitional  rocks  just  mentioned.  In  an  intrusion  at  Sale 
Fell,  near  Bassenthwaite,  quartz  occurs  partly  as  interstitial 
grains,  partly  in  micrographic  intergrowth,  and  the  rock 
shews  considerable  resemblance  to  the  original  kersantites  of 
Brittany.  The  last-named  rocks  are  sometimes  even-grained, 
sometimes  porphyritic  ('  porphy rites  micace'es'  of  Barrois). 

Mica-lamprophyres  are  known  also  from  the  Isle  of  Man 
(Peel  Castle),  Galloway1,  the  Cowal  district  of  Argyllshire, 
Invernessshire  (Farley  near  Beauly "),  and  some  parts  of  Ireland. 

An  augite-bearing  minette3  seems  to  be  one  of  the  com- 
monest types  of  lamprophyres.  It  is  seen  in  Cornwall  (Tre- 
lissick  Creek,  etc.),  in  the  Channel  Islands  (Doyle  Monument, 
Guernsey),  and  at  numerous  foreign  localities  (e.g.  Plauen'scher 
Grund,  near  Dresden).  With  more  abundant  augite  (e.g. 
Weinheirn  in  the  Odenwald)  it  passes  into  the  augite-vogesites. 
The  typical  vogesites  of  the  Vosges,  etc.,  have  sometimes  augite, 
sometimes  hornblende,  as  the  dominant  coloured  constituent, 
the  principal  felspar  being  orthoclase. 

In  America  mica-lamprophyres  of  the  minette  type  have 
been  described  from  Coanicut  Island,  R.L4,  Franklin  Furnace, 
N.J.5,  Ndtre  Dame  Bay  in  Newfoundland6  (with  accessory 
augite  and  hornblende),  and  the  Sweet  Grass  Hills T  (with 
augite)  and  Little  Belt  Mountains8,  Mont.  The  kersantite 
type  is  recorded  from  the  Sierra  Nevada  region  of  California 
(Mariposa,  Hamilton,  etc.),  and  an  augite-vogesite  from  the 
Black  Hills  of  Dakota9. 

1  Teall,  Mem.  Geol.  Sur.,  Silur.  Rocks  Scot.  (1899)  628,  629. 

2  Home,  M.  M.  (1886)  vii,  p.  iv. 

3  Cohen  (3),  pi.  xxiv,  fig.  4  (Weinheim)  ;  Berwerth,  Lief,  in  (Schwar- 
zenbach,  Odenwald). 

4  Pirsson,  A.  J.  S.  (1893)  xlvi,  374. 

5  Iddings,  in  Diller,  236-239. 

6  Wadsworth,  A.  J.  S.  (1884)  xxviii,  99,  100. 

7  Weed  and  Pirsson,  A.  J.  S.  (1895)  1,  313. 

8  Pirsson,  2Qth  Ann.  Rep.  U.  S.  G.  S.,  part  in.  (1900)  526-531,  pi. 

LXXVI,  A. 

9  J.  D.  Irving,  Ann.  N.  Y.  Acad.  Sci.  (1899)  xii,  287. 

10—2 


148  HORNBLENDE-LAMPROPHYRES. 

In  the  north-eastern  United  States  and  in  Canada  horn- 
blende-lamprophyres  of  the  camptonite  type  are  widely  dis- 
tributed. The  name  was  first  applied  by  Kosenbusch  to  rocks 
described  by  Hawes1  from  Camp  ton  Falls,  N.H.,  while  closely 
similar  rocks  are  found  near  Montreal2,  at  Summit  Station3  and 
Mount  Ascutney4,  Vt.,  at  several  points  on  the  Hudson  River 
highlands5  arid  in  the  Lake  Champlain  district6,  and  (with  less 
abundant  hornblende)  at  the  Forest  of  Dean  iron-mine,  N.Y.7 
In  all  these  idiomorphic  brown  hornblende,  usually  in  two 
generations,  is  the  chief  constituent,  felspar  is  subordinate, 
and  augite  is  at  most  an  accessory.  In  other  varieties  augite 
becomes  prominent  in  addition  to  the  dominant  hornblende. 
When  augite  predominates,  the  rock  may  be  termed  augite- 
camptonite,  but  such  rocks  shew  an  approach  to  diabase  by 
the  augite  losing  its  sharply  idiomorphic  habit. 

Hornblendic  lamprophyres,  some  more  or  less  closely  of 
the  camptonite  type,  are  found  in  various  British  localities. 
Some  of  the  Warwickshire  rocks  originally  described  as 
diorites  are  camptonites,  particularly  one  from  Marston  Jabet8. 
This  contains  abundant  brown  hornblende  in  idiomorphic 
elongated  crystals.  Some  of  these  rocks  carry  porphyritic 
augite,  and  some  contain  olivine.  Another  district  for  camp- 
tonites is  that  of  Beinn  Nevis  (Sgor  Chalum,  etc*),  and  rocks 
more  or  less  closely  allied  occur  in  Galloway  (Black  Gairy 
Hill10)  and  in  the  Cowal  district  of  Argyllshire11.  Some 
larnprophyre  sills  and  dykes  in  the  Assynt  district  of  Suther- 


1  A.  J.  S.  (1879)  xvii,  147-151 :  also  ladings,  in  Diller,  239,  240  ;  and 
see  Berwerth,  Lief.  n. 

2  Harrington,  Rep.  Geol.  Sur.  Can.  187^. 

3  Nason,  A.  J.  S.  (1889)  xxxviii,  229. 

4  Jaggar,  Bull.  No.  148,  U.  S.  G.  S.  (1897)  70. 

5  Kemp,  Amer.  Naturalist,  1888,  694-696,  pi.  xn. 

6  Kemp  and  Marsters,  Trans.  N.  Y.  Acad.  Sci.  (1891)  xi,  21,  22;  Bull. 
No.  107,  U.  S.  Geol.  Sur.  (1893)  29-32. 

7  Kemp,  A.  J.  ft.  (1888)  xxxv,  331,  332. 

8  Allport,  Q.  J.   G.  S.  (1879)  xxxv,  638,  639 ;  Watts,  Pr.  Geol.  Ass. 
(1898)  xv,  394  ;  Teall,  pi.  xxix,  fig.  2. 

,9  Teall,  Summary  of  Progress,  Geol.  Sur.  for  1898,  48. 

10  Teall,  Mem.  Geol'.  Surl,  Silur.  Rocks  Scot.  (1899)  pi.  xxv,  fig.  2. 

11  Teall,  Mem.  Geol.  Sur.  Scot.,  Geol.  Cowal  (1897),  116-118. 


FJORNBLENDE-LAMPROPHYRES. 


149 


land1,  except  for  the  colour  of  their  hornblende,  are  identical 
with  the  camptonite  type.  They  are  characterized  by  abundant, 
slender-twinned  crystals  of  hornblende,  sometimes  of  hollow 
shape  (fig.  33).  In  Ireland  hornblendic,  as  well  as  micaceous, 
lamprophyres  are  known  from  Galway,  the  Kaphoe  district,  the 
coast  of  Down,  etc.  Prof.  Watts'2  describes  one  from  Lettery, 
near  Clifden,  as  a  camptonite,  another  from  Clondermot,  near 
Raphoe,  as  a  vogesite,  and  most  of  those  in  Co.  Down  carry 
hornblende  in  addition  to  biotite. 


FlG.    33.       HORNBLEN1>E-LAMPROPH¥HE    (APPROACHING    CAMPXONITlfi),    FROM 
INTRUSIVE    SILL   IN    DlJRNESS   LlMESTONE,    LOCH    AgSYNT  ;    X  20. 

Shewing  phenocrysts  of  green  hornblende  in  a  panidiomorphic  ground- 
mass  of  plagioclase  and  hornblende,  with  a  little  magnetite  and  apatite 
[1687]. 


There  remain  the  monchiquites,  lamprophyric  rocks  of  very 
low  silica-percentage  and  with  a  peculiar  mineralogical  com- 
position. Felspar  is  absent,  and  instead  there  is  a  colourless 
isotropic  base  of  low  refractive  index,  which  was  originally 
regarded  as  a  glass,  but  in  some  cases  at  least  is  found  to  be 


1  Teall,  G.  M.  1886,  346-353. 

2  Guide,  53,  73-75. 


150  MONCHIQUITES. 

analcime1.  The  other  characteristic  minerals  are,  in  the 
typical  rocks,  olivine  and  augite,  or  in  some  varieties  horn- 
blende. Such  rocks  are  known  in  Portugal,  Brazil,  Arkansas2 
and  the  Lake  Champlain  district3.  Here  too  belong  the  rocks 
described  in  Montana  under  the  name  analcime-basalt4.  A 
good  example,  with  abundant  analcime,  has  also  been  described 
from  near  Cripple  Creek,  Colorado5.  The  only  British  rocks  of 
this  type  yet  known  are  those  described  by  Dr  Flett6  from  the 
Orkney  Islands.  Here  too  occurs  the  rarer  Fourche  type, 
devoid  of  olivine,  which  is  found  in  Arkansas7  and  the  Lake 
Champlain  district8.  .  The  Ouachita  type,  from  the  former  area, 
is  also  without  olivine,  and  is  characterized  by  biotite  as  its 
dominant  ferro-magnesian  mineral9. 

It  is  to  be  observed  that  these  various  types,  as  well  as  the 
camptonites,  are  met  with  in  association  with  nepheline-syenites 
and  allied  rocks,  while  the  more  usual  mica-lamprophyres 
occur  in  connection  with  granites,  etc. 

1  Pirsson,  Journ.  of  Geol  (1896)  iv,  679-690  ;  Evans,  Q.  J.  G.  S.  (1901) 
Ivii,  38-53. 

2  J.  F.  Williams,  Ign.  Rocks  ArTc.,  vol.  ii  of  Rep.  Geol.  Sur.  Ark.  for 
1890,  151-157,  290-295,  353. 

3  Kemp  and  Marsters,  Trans.  N.  Y.  Acad.  Sci.  (1891)  xi,  22,  23 ;  Bull. 
No.  107,  U.  S.  Geol.  Sur.  (1893)  32-35. 

4  Lindgren,  Proc.  Calif.  Acad.  Sci.  (1896)  iii,   51  (Highwood  Mts) ; 
Weed  and  Pirsson,  Bull.  No.  139,  U.  S.  G.  S.  (1896)  114-117  (Castle  Mt.) ; 
Pirsson,  20th  Ann.  Rep.  U.  S.  G.  S.  part  in  (1900),  543-550  (Little  Belt 
Mts). 

5  Cross,  Journ.  of  Geol.  (1897)  v,  684-693. 

6  Trans.  Roy.  Soc.  Edin.  (1900)  xxxix,  887-896. 

7  J.  F.  Williams,  I.e.  107,  108,  290. 

8  Kemp  and  Marsters,  I.e.  35,  36. 

9  Kemp  in  Ign.  Rocks  Ark.  394-398. 


C.     VOLCANIC   ROCKS. 


UNDER  this  head  we  shall  treat  only  the  solid  rocks  of 
volcanic  origin  (lavas),  reserving  the  fragmented  products  of 
volcanic  action  for  the  sedimentary  group.  With  the  true 
extruded  lava-flows  will  be  included  similar  rocks  occurring 
in  the  form  of  dykes,  etc.,  in  direct  connection  with  volcanic 
centres,  the  common  feature  of  all  being  that  they  have  con- 
solidated from  fusion  under  superficial  conditions,  i.e.  by  com- 
paratively rapid  cooling  under  low  pressure.  This  mode  of 
origin  has  given  the  rocks  as  a  whole  characters  which  place 
them  in  contrast  with  the  plutonic  group,  while  the  types 
treated  above  under  the  head  of  'hypabyssal'  have  in  some 
respects  intermediate  characters.  Many  volcanic  outpourings 
have  undoubtedly  been  submarine,  and  when  these  have  taken 
place  under  a  great  depth  of  water  the  products  may  be  ex- 
pected to  approximate  in  some  measure  to  the  characters  of 
rocks  of  deep-seated  origin.  In  general,  however,  the  contrast 
between  volcanic  and  plutonic  types  of  structure  is  well  marked. 

The  presence  of  a  glassy  (or  devitrified)  residue,  though 
not  peculiar  to  volcanic  rocks,  is  highly  characteristic  of  them, 
and  especially  of  the  more  acid  members.  Other  features 
characteristic  of  lavas,  though  not  confined  to  them,  are  the 
vesicular  and  amygdaloidal  structures,  and  the  various  fluxion- 
phenomena,  including  flow-lines,  parallel  orientation  of  pheno- 
crysts,  banding,  drawing  out  of  vesicles,  etc. 

The  great  majority  of  the  volcanic  rocks  have  a  porphyritic 
structure,  i.e.  their  constituents  belong  to  two  distinct  periods 
of  consolidation,  the  earlier  represented  by  the  porphyritic 


152  CHARACTERISTICS    OF   VOLCANIC    ROCKS. 

crystals  or  'phenocrysts",  and  the  later  by  the  'ground-mass,' 
which  encloses  them,  and  commonly  makes  up  the  bulk  of  the 
rock.  This  ground-mass  may,  and  usually  does,  include  some 
glassy  residue  or  'base':  if  the  ground  is  wholly  glassy,  we 
have  what  is  termed  the  'vitrophyric'  structure.  The  same 
mineral  may  occur  both  among  the  phenocrysts  and  as  a  con- 
stituent of  the  ground-mass.  When  such  a  recurrence  is  found, 
the  crystals  of  the  earlier  generation  are  distinguished  from 
those  of  the  later  by  their  larger  size,  often  by  their  more  per- 
fect idiomorphism,  and  in  some  cases  by  fracture,  corrosion,  or 
other  evidence  of  vicissitudes  in  their  history.  The  two  periods 
of  consolidation  are  styled  by  Rosenbusch  the  'intratelluric' 
and  the  'effusive,'  the  former  being  considered  as  the  result  of 
crystallization  prior  to  the  pouring  out  of  the  lava,  and  so 
under  more  or  less  deep-seated  conditions.  When  we  speak 
of  the  consolidation  of  a  lava  at  the  earth's  surface,  we  must 
be  understood  to  refer  to  the  ground-mass  of  the  rock.  In 
some  few  types  of  lavas  the  phenocrysts  fail  altogether,  and 
the  effusive  period  is  the  only  one  represented. 

The  various  types  will  be  grouped  under  families,  to  be 
taken  roughly  in  order,  beginning  with  the  most  acid.  It  is 
customary  to  speak  of  the  several  families  of  lavas  as  answer- 
ing to  the  commonly  recognized  families  of  the  plutonic  rocks 
— the  rhyolites  to  the  granites,  the  trachytes  to  the  syenites, 
etc. — but  such  a  correspondence  cannot  be  followed  out  with 
great  exactness.  It  is  certain  that  a  given  rock-magma  may 
result  in  very  different  mineral-aggregates  according  as  its 
consolidation  is  effected  under  deep-seated  or  under  surface 
conditions ;  and  in  the  latter  case,  moreover,  much  of  the 
rock  produced  may  consist  of  unindividualised  glass. 

It  is  more  especially  in  the  volcanic  rocks  that  the  Con- 
tinental petrologists  have  insisted  upon  a  division  into  an 
'older'  and  a  'younger'  series  ('palseo  volcanic'  and  'neo- 
volcanic'),  an  arbitrary  line  being  drawn  between  the  pre- 
Tertiary  lavas  and  the  Tertiary  and  Recent.  This  distinction 
is  rejected  by  the  British  school,  and  will  find  no  place  in  the 

1  This  convenient  term,  due  to  Prof.  Iddiugs,  will  be  adopted  here. 
Mr  Blake  has  proposed  the  word  'inset,'  as  corresponding  to  the  Ger. 
'  Einsprengling.' 


NOMENCLATURE   OF    VOLCANIC    ROCKS.  153 

following  pages1.  The  simplified  grouping  of  the  volcanic 
rocks  by  their  essential  characters,  without  reference  to  their 
age  or  supposed  age,  involves  some  modification  of  the  double 
nomenclature  in  use  among  the  German  and  French  writers. 
The  names  employed  by  them  for  the  younger  lavas  only  will 
here  be  extended  to  all  rocks  of  the  same  character,  irrespective 
of  their  geological  antiquity.  The  names  applied  by  the  Con- 
tinental writers  to  the  pre-Tertiary  lavas  have  also  been  used 
habitually  for  hypabyssal  rock-types,  and  may  now  be  restricted 
to  these  latter.  Some  of  them  (quartz-porphyry,  porphyrite, 
diabase,  etc.)  we  have  already  used  in  this  sense. 

1  On  this  question  see  Sci.  Progr.  (1894)  ii,  48-63. 


CHAPTER  XL 

RHYOLITES. 

IN  the  rhyolite  family  we  include  all  the  truly  acid  lavas  ; 
rocks  of  porphyritic  or  vitrophyric  structure,  in  which  alkali- 
felspars  and  usually  quartz  figure  as  the  chief  constituent 
minerals.  By  the  older  writers  most  of  these  rocks  were  in- 
cluded, with  others,  under  the  large  division  'trachyte.'  The 
present  family  was  separated  by  von  Richthofen  with  the  name 
'rhyolite,'  expressing  the  fact  that  flow-structures  are  commonly 
prominent  in  the  rocks.  Roth  used  the  term  'liparite'  in 
nearly  the  same  sense.  The  Continental  petrographers,  fol- 
lowing their  regular  principle,  use  these  names  for  the  Tertiary 
and  Recent  acid  lavas  only,  the  older  (pre-Tertiary)  being  more 
or  less  arbitrarily  separated  and  designated  by  other  names 
(quartz-porphyry,  porphyry,  etc^)\  and  some  English  geologists 
have  tacitly  adopted  a  like  division,  calling  the  older  rhyolites, 
which  have  often  suffered  various  secondary  changes,  quartz- 
felsites,  felsites,  etc. 

Some  petrologists  distinguish  between  potash-  and  soda- 
rhyolites,  according  to  the  predominance  of  one  or  the  other 
of  the  alkalies  ;  but  in  fine-textured  or  glassy  rocks  this 
difference  does  not  always  express  itself  in  the  minerals 
evident.  There  is,  however,  a  peculiar  group  of  acid  lavas 
very  rich  in  alkalies,  and  especially  in  soda:  these  rocks, 
the  'pantellarites'  of  Forstner,  contain  special  characteristic 
minerals. 

We  shall  consider  briefly  the  characters  of  the  phenocrysts 
or  enclosed  crystals  and  of  the  ground-mass.  In  some  rhyolites 


PHENOCRYSTS   OF   RHYOLITES.  155 

the  phenocrysts  occur  only  sparingly,  or  may  even  fail  alto- 
gether. 

Phenocrysts.  Among  the  phenocrysts  or  porphyritically 
enclosed  crystals  of  the  rhyolites,  the  most  constant  are  alkali- 
felspars;  both  orthoclase  (including  sanidine)  in  tabular  or 
columnar  crystals,  simple  or  twinned,  and  an  acid  plagioclase, 
ranging  from  albite  to  oligoclase,  in  tabular  crystals  with  the 
usual  twin-lamellation.  A  parallel  intergrowth  of  the  mono- 
clinic  and  triclinic  species  is  occasionally  found.  The  felspars 
often  contain  glass-1  and  gas-cavities,  but  rarely  fluid-pores : 
such  minerals  as  apatite,  magnetite,  biotite,  etc.,  may  be 
sparingly  enclosed.  Certain  rocks  specially  rich  in  soda  (pan- 
tellarites,  etc.)  have  anorthoclase. 

Quartz,  when  present,  occurs  in  dihexahedral  crystals, 
often  corroded  and  with  inlets  of  the  ground-mass.  Besides 
occasional  inclusions  of  minerals  of  early  consolidation,  it 
contains  glass-  but  rarely  fluid-cavities. 

The  more  basic  silicates  are  not  present  in  great  abundance. 
The  most  usual  is  biotite  in  deep-brown  hexagonal  flakes,  with 
only  occasional  inclusions  of  apatite,  zircon,  or  magnetite.  A 
greenish  augite  with  octagonal  cross-section  may  be  present, 
but  brown  hornblende  is  much  less  common.  The  pantellarites 
have  the  brown  triclinic  amphibole  cossyrite,  with  intense  ab- 
sorption and  pleochroism. 

The  most  usual  iron-ore  is  magnetite,  but  it  is  rarely 
abundant.  Needles  of  apatite  and  minute  crystals  of  highly 
refringent  and  birefringent  zircon  may  also  occur  in  small 
quantity.  In  rarer  cases  garnet  is  found  instead  of  a  ferro- 
magnesian  bisilicate. 

Ground-mass  and  structures.  The  rhyolites  exhibit 
in  their  ground-mass  a  great  variety  of  texture  and  structure. 
The  texture  may  be  wholly  or  partly  glassy ;  or  cryptocrystalline, 
often  with  special  structures ;  or,  again,  evidently  crystalline, 
though  on  a  minute  scale.  Further,  these  several  varieties  of 
ground-mass  may  be  associated  in  the  same  rock  and  in  the 
same  microscopical  specimen.  Fluxion  is  frequently  marked 
by  banding,  successive  bands  being  of  different  textures,  so 

1  Cohen  (3),  pi.  ix,  fig.  4. 


156 


GLASSY   GROUND-MASS   IN   RHYOLITES. 


that  thin  layers  of  glassy  and  stony  or  spherulitic  nature  altern- 
ate with  one  another. 

The  vitreous  type  of  ground-mass  alone  is  found  in  the 
obsidians1.  These  rocks,  colourless  or  very  pale  yellow  in 
thin  slices,  afford  good  examples  of  structures  common  to  all 


FIG.  34.     GLASSY  KHYOLITE  (OBSIDIAN),  TELKIBANYA,  NEAR 
SCHEMNITZ,  HUNGARY  ;    x  20. 

Shewing  sinuous  flow-lines  traversed  by  a  system  of  curving  perlitic 
fissures  [G.  329]. 

the  natural  glasses ;  especially  the  perlitic  cracks  (fig.  34), 
produced  by  contraction  of  the  homogeneous  material2,  and 
the  vesicular  structure  due  to  the  rock-magma  having  been 
distended  by  steam-bubbles.  In  extreme  cases  the  cavities  are 
so  numerous  as  to  make  up  the  chief  part  of  the  volume  of  the 
rock,  and  we  have  the  well-known  pumice  (Fr.  ponce,  Ger. 
Bimstein).  The  vesicles  are  commonly  elongated  in  the 
direction  of  flow,  and  may  even  be  drawn  out  into  capillary 

1  The  less  common  glassy  rocks  of  the  trachyte  and  plionolite  family 
and  of  the  dacites  are  also  termed  obsidian.     They  are  not  easily  distin- 
guished from  the  rhyolite-glasses.     Some  of  the  rocks  styled  pitchstones 
are  lavas  of  the  obsidian   type,  usually  of  acid  composition   (e.g.   the 
4  Meissen  pitchstones,'  in  Saxony). 

2  Cohen  (3),  pi.  LXXI,  figs.  1,  2. 


CRYSTALLITES    IN   RHYOLITES. 


157 


tubes.     In  the  older  lavas  vesicles  are  usually  filled  by  second- 
ary products,  and  become  amygdules. 

In  many  cases  a  ground-mass  consisting  essentially  of  glass 


B 


FIG.  35.     CRYSTALLITES  IN  OBSIDIAN. 

A.  Margarites,  Obsidian  Cliff,  Yellowstone  Park;  x  400  [477]. 
B.  Trichites,  Telkibanya,  Hungary  ;  x  100  [G.  327].  C.  Longulites 
and  swallow-tailed  crystallites,  Hlinik,  Hungary;  x  200  [G.  70]. 
D.  Flow-structure  marked  by  arrangement  of  twisted  trichites,  Pra- 
bacti,  Java ;  x  200  [G.  64]. 

encloses  minute  bodies  known  as  crystallites  (fig.  35),  which 
may  be  regarded  as  embryonic  crystals1.  They  have  definite 
forms,  but  no  perfect  crystal  boundaries,  and  the  more  rudi- 
mentary types  cannot  be  subjected  to  optical  tests  to  deter- 
mine their  nature.  The  simplest  effort  at  individualisation 
from  the  vitreous  mass  results  in  globulites,  minute  spherical 
bodies  without  action  on  polarized  light.  They  occur  in 
profusion  in  many  obsidians,  either  uniformly  distributed  or 
aggregated  into  cloudy  patches  (cumulites).  From  the  partial 
coalescence  of  a  series  of  globulites,  arranged  in  a  line,  result 
margarites2,  resembling  strings  of  pearls.  A  high-power 

1  See  Eutley,  H.  M.  (1891)  ix,  261-271,  and  plate;  Zirkel,  Micro.  Petr. 
Fortieth  Parallel,  pi.  ix,  figs.  1-4  ;  Rosenbusch-Iddings,  pi.  n,  in. 

2  Cohen  (3),  pi.  vi. 


158  CRYSTALLITES:    DEVITRIFICATION. 

objective  (say  -§-  inch)  is  often  necessary  to  resolve  this  beaded 
structure.  Long  threads  of  this  nature  may  extend  in  the 
direction  of  flow  but  with  numerous  little  twists1.  Similar 
threads  with  curved  hair-like  form,  known  as  trichites,  often 
occur  in  groups  originating  in  a  common  nucleus.  These 
bodies,  in  which  a  beaded  structure  may  or  may  not  be 
observable,  often  seem  to  belong  to  a  stage  of  development 
later  than  the  cessation  of  flowing  movement  in  the  mass2. 
The  small  rod-like  bodies  known  as  longulites,  sometimes 
slightly  clubbed  at  the  ends3,  may  be  regarded  as  built  up  by 
the  complete  union  of  rows  of  globulites.  They  often  occur  in 
crowds,  with  a  marked  arrangement  parallel  to  the  direction 
of  flow.  The  transition  from  margarites  to  longulites  is  often 
seen,  some  of  the  little  rods  resolving  into  beaded  strings, 
while  others  do  not.  The  larger  crystallitic  bodies  termed 
microlites  are  possibly  to  be  conceived  as  built  up  from  longu- 
lites. Various  incomplete  stages  may  be  observed,  the  ends 
of  the  imperfect  microlites  having  a  brush-like  form  (scopulites 
of  Rutley)  or  being  forked  in  swallow-tail  fashion.  Fully 
developed  microlites  have  an  elongated  form,  and  are  indeed 
small  crystals  giving  the  optical  reactions  proper  to  the  mineral 
(felspar,  augite,  hornblende,  etc.}  of  which  they  consist. 

An  original  cryptocrystalline  or  'microfelsitic'  ground-mass 
is  found  in  some  rhyolites,  though  it  seems  to  be  more  charac- 
teristic of  intrusive  types  (approaching  what  we  have  styled 
quartz-porphyries)  than  of  true  surface  lavas.  It  consists  in 
a  granular  mixture  of  felspar  and  quartz  on  so  minute  a  scale 
that  the  individual  grains  cannot  be  resolved  in  a  thin  slice. 
There  is  no  doubt,  however,  that  in  many  old  acid  lavas  a 
cryptocrystalline  ground-mass  has  resulted  from  the  devitrifi- 
cation (Ger.  Entglasung)  of  a  rock  originally  vitreous.  The 
process  has  often  begun  along  perlitic  fissures,  or  flow-lines, 
and  the  successive  stages  are  beautifully  displayed  in  such  rocks 
as  the  Permian  rhyolites  ('pitchstones')  of  Meissen  in  Saxony. 
No  single  criterion  can  be  set  up  for  distinguishing  an  original 
from  a  secondary  cryptocrystalline  structure.  In  a  rock 

1  Zirkel,  Micro.  Petrogr.  Fortieth  Parallel  (187G),  pi.  ix,  figs.  3,  4. 

2  Ibid.  figs.  1,  2. 

3  Fouqu<§  and  L£vy,  pi.  xvi,  fig.  2. 


MICROCRYSTALLINE   AND   SPHERULITIC   STRUCTURES.      159 

otherwise  fresh,  however,  there  will  generally  be  no  reason 
to  suspect  devitrification ;  while,  on  the  other  hand,  the 
presence  of  perlitic  cracks  is  often  taken  to  indicate  that 
the  rock  in  which  they  occur  was  originally  glassy1. 

A  microcrystalline  (as  distinguished  from  cryptocrystalline) 
ground-mass  is  not  very  prevalent  in  true  acid  lavas,  but  may 
occur  as  bands  alternating  with  glassy  or  microspherulitic 
bands,  often  on  a  small  scale.  When  an  evident  microcrystal- 
line structure  has  been  set  up  as  a  secondary  alteration,  it 
probably  indicates,  as  a  rule,  something  more  than  the  merely 
physical  change  of  devitrification.  It  is  often  connected  with 
an  introduction  of  silica  from  an  external  source,  and  in  the 
resulting  microcrystalline  mosaic  quartz  often  plays  a  more 
important  part  than  it  would  do  in  a  normal  igneous  rock. 
In  some  of  the  partly  silicified  Ordoyician  rhyolites  of  West- 
morland a  secondary  quartz-mosaic  still  shews  clear  indication 
of  former  perlitic  cracks,  outlined  by  dust,  as  well  as  the 
characteristic  banding.  In  these  rocks,  too,  silicification  has 
sometimes  affected  not  only  the  ground-mass  but  the  felspar 
phenocrysts. 

Spherulitic  and  allied  structures.  The  spherulitic 
growths  which  are  common  in  many  acid  lavas  may  be  con- 
veniently divided  into  the  larger  and  the  smaller.  Under  the 
former  head  we  have  spherulites,  often  isolated,  with  diameters 
ranging  from  a  fraction  of  an  inch  to  several  inches.  They  are 
best  studied  in  certain  obsidians,  where  they  are  usually  of 
distinctly  globular  form  and  with  well-defined  boundary.  The 
examples  which  have  been  most  carefully  examined,  and  may 
be  taken  as  typical,  consist  mainly  of  extremely  delicate  fibres 
of  felspar,  arranged  radially  and  on  the  whole  straight,  but 
often  forked  or  branching2.  In  the  spherulites  of  perfectly 
fresh  rocks  the  space  between  the  fibres  is  found  to  be  occupied 
in  great  part  by  aggregates  of  tridymite.  In  older  spherulites, 
where  tridymite  is  not  recognized,  quartz  may  perhaps  be 

1  Some  American  writers  have  used  the  name  '  aporhyolite  '  for  such 
devitrified  rhyolites. 

2  See  Cross,  Bull.  Phil.  Soc.  Washington  (1891),  xi,  411-414 ;  Iddings, 
ibid.  445-464,  with  plates.     Similar  structures  occur  in  dykes  on  Druim 
an  Eidhne,  near  Loch  Coruisk,  Skye  :  see  Judd,  Q.  J.  G.  S.  (1893)  xlix, 
pi.  n,  in. 


160 


SPHERULITES    AND    LITHOPHYSES. 


considered  to  represent  it.  In  any  case  the  structure  is  to  be 
made  out  only  in  carefully  prepared  and  very  thin  slices.  It 
may  often  be  observed  that  the  flow-lines  of  the  lava  pass 
undisturbed  through  the  spherulites,  indicating  that  the  latter 


FIG.  36.     OBSIDIAN,  VULCANO,  LIPARI  Is.  ;    x  20. 

The  glassy  matrix  encloses  isolated  spherulites,  with  some  tendency 
to  coalesce  in  bands  following  the  direction  of  flow.  The  flow-lines  pass 
uninterruptedly  through  the  spherulites  [1785]. 

crystallized  after  the  cessation  of  movement.  Spherulites  are 
often  developed  along  particular  lines  of  flow,  and  may  coalesce 
into  bands  (fig.  36). 

These  larger  spherulites  shew  many  special  peculiarities  in 
different  examples.  Sometimes  their  outward  extension  has 
been  effected  in  two  or  more  stages,  which  are  marked  by 
a  change  in  the  character  of  the  growth.  Again,  curious 
phenomena  arise  from  the  formation  of  shrinkage-cavities 
(lithophyses)  in  connection  with  spherulites.  Some  remarkable 
examples  of  lithophyses  have  been  described  from  the  Yellow- 
stone Park1  and  other  districts  in  the  United  States2,  from 

1  Iddings,  Obsidian  Cliff,  in  7th  Ann.  Rep.  U.  S.   Geol.  Surv.  (1888) 
265,  266,  pL  xn-xiv ;  A.  J.  S.  (1887)  xxxiii,  36-43. 

2  Nathrop,  Colorado  ;  see  Cross,  Proc,  Colo,  Sci,  Soc.  (1886)  62-66. 


PYROMERIDES :   MICROSPHERULITIC   STRUCTURE.      161 

Hungary,  and  from  Lipari1.  A  peculiar  feature  is  the  oc- 
currence in  the  hollows  of  perfect  crystals  of  the  iron-olivine 
(fayalite),  as  well  as  aggregates  of  tridymite,  and  in  some  cases 
crystals  of  garnet,  topaz,  etc.  The  complex  forms  of  these 
lithophyses  can  be  realised  only  from  specimens  or  figures. 
They  must  be  distinguished  from  ordinary  ovoid  vesicles. 

The  large  spherulites  are  in  some  cases  only  skeleton- 
structures,  the  divergent  rays  being  embedded  in  glass.  Such 
skeleton-spherulites,  in  a  devitrified  matrix,  have  been  described 
by  Prof.  Cole2  in  the  'pyrome'rides'  of  Wuenheim,  in  the 


Examination  of  the  older  acid  lavas  shews  that  the  large 
spherulites  are  specially  susceptible  to  certain  chemical  changes. 
They  are  often  found  partly  or  totally  replaced  by  flint  or 
quartz,  while  their  insoluble  decomposition-products  remain 
as  roughly  concentric  shells  of  a  chloritic  or  pinitoid  sub- 
stance. Again,  a  central  hollow  is  often  found,  and  it  is  not 
always  clear  whether  this  is  due  entirely  to  decomposition  or 
partly  represents  an  original  lithophysal  cavity3,  nodular 
structures  originating  in  both  ways  being  represented  in 
many  districts. 

The  very  minute  spherulites  commonly  occur  in  large 
numbers,  closely  packed  together,  so  as  to  constitute  the  chief 
bulk  of  particular  bands,  or  even  of  the  whole  ground-mass  of 
the  rock.  This  is  the  microspherulitic  structure4.  The  true 
nature  of  these  very  minute  bodies,  as  composed  of  fine  fibres 
of  felspar  with  quartz,  is  a  matter  rather  inferred  than  seen  in 
any  given  case  ;  but  the  radiate  growth  is  detected  by  means 
of  the  'black  cross,'  which  each  individual  spherulite  shews 
between  crossed  nicols  (figs.  37,  38,  A).  These  minute  spheru- 
lites seem  to  be  much  less  readily  destroyed  than  the  larger 
ones.  The  axiolites  of  Zirkel5  seem  to  be  of  the  nature  of 

1  Cole  and  Butler,  Q.  J.  G.  S.  (1892)  xlviii,  438-443,  pi.  xn  ;  John- 
ston-Lavis,  G.  M.  1892,  488-491. 

2  G.  M.  1887,  299-303. 

3  See  especially  Cole,  Q.  J.  G.  S.  (1886)  xlii,  183-190  ;  (1892)  xlviii, 
443-445  ;  Parkinson,  ibid.  (1901)  Ivii.  211-225,  pi.  vin. 

4  See  Teall,  pi.  xxxvra. 

5  Micro.  Petr.  Fortieth  Parallel,  pi.  vi,  fig.  2.     But   compare  Cole, 
M.  M.  (1891)  ix,  271-274. 

H.  P.  11 


162 


PHENOCRYSTS   OF   MICROPEGMATITE. 


elongated  spherulites,  the  fibres  radiating  not  from  a  point 
but  from  an  axis  (fig.  38,  A )  ;  or  they  may  be  conceived  as 


FlG.    37.       MlCROSPHERULITIC    RHYOLITE,    GREAT   YARLSIDE, 

WESTMORLAND  ;    x  20,  CHOSSED  NICOLS. 
Each  little  spherule  shews  a  black  cross  [1813]. 

representing  the  coalescence  of  a  row  of  minute  spherulites 
(Of.  %.  36). 

Any  evident  micrographic  structure  is  not  common  in  the 
ground-mass  of  rhyolites,  though  bands  or  streaks  having  this 
character  are  sometimes  found.  A  curious  feature,  first  de- 
scribed by  Iddings  in  some  obsidians  from  the  Yellowstone 
Park1  and  rhyolites  from  the  Eureka  district  of  Nevada2, 
is  the  occurrence  of  porphyritic  '  granophyre  groups '  or 
micropegmatite  phenocrysts  in  a  glassy,  cryptocrystalline,  or 
microcrystallihe  ground-mass  (see  fig.  38,  H).  In  these  the 
quartz  is  subordinate  to  the  felspar  in  quantity,  and  the 
micrographic  groups  often  shew  the  crystal-boundaries  of  the 
latter  mineral.  As  a  rule,  however,  there  are  several  felspar 
crystals  grouped  together,  the  whole  permeated  by  wedges  of 
quartz,  and  the  outline  is  complex  or  rather  irregular. 

1  1th  Ann.  Eep.  U.  S.  Geol.  Sur.  (1888)  274-276,  pi.  xv. 

2  Monog.  xx  U.  S.  Geol.  Sur.  (1893)  375,  pi.  v,  fig.  2. 


MICROPCECILITIC   STRUCTURE.  163 

A  structure  met  with  in  the  ground  of  some  rhyolites,  and 
in  certain  bands  of  laminated  rhyolites,  differs  essentially  from 


mpc. 


ax. 


FIG.  38.     SPECIAL  STRUCTURES  IN  RHYOLITES,    x  20 ;   CROSSED  NICOLS. 

A.  Falls  of  Gibbon  River,  Yellowstone  Park  :  different  bands, 
following  the  flow-lines,  shew  micropoecilitic  (rnpc),  axiolitic  (ax),  and 
microspherulitic  (sp)  structures  [1430].  B.  Goodwick,  near  Fishguard, 
Pembrokeshire  ;  shewing  micropegmatite  phenocrysts  in  a  finely  micro- 
crystalline  ground-mass  [2289]. 

the  micrographic,  in  that  it  indicates  the  successive,  instead 
of  simultaneous,  crystallization  of  the  two  constituent  minerals. 
Minute  felspar  crystals  with  no  orderly  arrangement  are  en- 
closed in  little  ovoid  or  irregular  areas  of  quartz,  the  whole  of 
the  quartz  in  such  a  little  area  being  in  crystalline  continuity. 
This  structure  reproduces  on  a  minute  scale  the  ophitic  and 
pcecilitic  structures  presented  by  different  minerals  in  other 
rocks,  and  Prof.  G.  H.  Williams  adopted  for  it  the  term  micro- 
pcecilitic1  (fig.  38,  A). 

An  original  holocrystalline  texture  on  other  than  a  minute 
scale  is  rarely,  if  ever,  met  with  in  true  rhyolites.  The 
*  nevadite '  of  Richthofen  is  exceptional  in  that  the  ground-mass 

1  Journ.  Geol.  (1893),  i,  176-179. 

11—2 


164  OBSIDIANS. 

is  quite  subordinate  in  quantity  to  the  crowded  phenocrysts, 
but  this  ground-mass  is  commonly  glassy.  In  part,  at  least, 
these  rocks  belong  to  the  dacites  rather  than  the  rhyolites. 

Leading  types.  The  glassy  type  (obsidian)  is  exempli- 
fied by  many  of  the  rhyolites  of  Iceland  and  of  Lipari1 
(fig.  36)  ;  and  in  the  latter  locality  pumice  is  extensively 
developed  (Monte  Chirica).  The  Hungarian  rhyolites  are  not 
usually  obsidians,  but  some  good  examples  occur  (Telkibanya) 2 
with  a  rich  variety  of  crystallites  (fig.  35).  Other  well-known 
obsidians  come  from  Ascension  Is.,  Mexico,  and  the  Yellow- 
stone Park.  The  rock  of  Obsidian  Cliff'3  in  the  last-named 
district  frequently  contains  spherulites  of  some  size,  isolated 
or  in  bands,  and  remarkable  chambered  lithophyses,  in  which 
occur  nests  of  tridymite  and  little  crystals  of  the  iron-olivine 
(fayalite).  Very  similar  phenomena  have  been  described  from 
Lipari  (Kocche  Rosse) 4,  and  some  of  the  Hungarian  lavas  also 
contain  small  lithophyses,  often  of  hemispherical  form,  cut  off 
by  the  fluxion-banding  of  the  lava.  It  was  there  that  these 
curious  structures  were  first  observed  by  von  Rlchthofen 
(Telkibanya,  Goncz,  etc.). 

The  more  widely  distributed  types  of  rhyolites  may  be 
studied  in  rich  variety  from  the  Tertiary  volcanic  districts  of 
Schemnitz  in  Hungary,  of  the  Lipari  group,  of  the  Western 
States  of  America,  etc.  They  differ  in  the  nature  of  their 
phenocrysts  and  in  the  structure  of  their  ground-mass.  Many 
of  them  have  a  strongly  marked  banded  structure,  successive 
narrow  bands,  a  fraction  of  an  inch  wide,  being  of  different 
textures  or  structures  (glassy,  microspherulitic,  axiolitic, 
microcrystalline,  micropoecilitic).  The  most  usual  ferro-mag- 
nesian  mineral  is  biotite,  but  it  is  never  plentiful. 

When  spherulitic  structures  are  present  they  may  be  on 
a  more  or  less  minute  scale.  Some  flows  in  the  Schemnitz 
district  are  built  up  almost  wholly  of  very  diminutive  spheru- 

1  Cohen  (3),  pi.  LXXII,  fig.  3. 

2  Cohen  (3),  pi.  LXXI,  fig.  2. 

3  Iddings,  1th  Ann.  Eep.   U.  S.  Geol.  Sur.  249-295  ;  and  in  Diller, 
151-160,  pi.  xxv-xxvn. 

4  Cole  and  Butler,  Q.  J.  G.  S.  (1892)  xlviii,  438-445 ;  Johnston-Lavis, 
G.  M.  1892,  488-491. 


AMERICAN   TERTIARY    RHYOLITES.  165 

lites1,  each  giving  a  perfect  black  cross  (Telkibanya,  Sarospatak, 
Eisenbach,  etc.).  This  microspherulitic  type  is  also  repre- 
sented among  the  rhyolites  of  the  Yellowstone  Park2.  In  the 
typical  '  perli tes '  of  the  Schemnitz  district  the  individual 
spherulites  are  larger,  with  well-marked  radial  fibrous  structure 
and  globular  form,  sharply  bounded,  often  by  perlitic  fissures 
(Hlinik,  etc.).  These  contrast  with  a  type  in  which  the 
spherulites  have  an  irregular  outline,  interlocking  with  one 
another  or  sending  out  processes  into  a  glassy  matrix. 

ZirkeP  described  from  Nevada  rhyolites  (including  ob- 
sidians) shewing  a  remarkable  variety  in  the  character  of 
their  ground-mass.  Others,  from  the  Eureka  district,  have 
been  described  by  Iddings4.  These  carry  a  little  biotite.  In 
examples  described  by  the  same  author5  from  New  Mexico 
(Tewan  Mts)  the  ferro-magnesian  mineral  is  augite.  In  these 
rocks  plagioclase  felspar  is  wanting  :  some  contain  spherulites 
and  lithophyses.  Rhyolites  from  Ouster  County,  Colorado, 
have  no  coloured  constituent  except  a  little  red  garnet6.  The 
ground-mass  is  usually  microcrystalline  to  cryptocrystalline, 
but  sometimes  spherulitic.  Biotite-bearing  rhyolites  with 
porphyritic  quartz  occur  in  the  Tin  tic  Mts,  Utah7.  Some 
varieties  in  the  Lassen's  Peak  district.  California,  are  highly 
spherulitic8.  Examples  from  the  Black  Hills  of  Dakota  have 
been  described  by  Caswell9,  but  Iddings10  classes  some  of 
these  rocks  as  dacites. 

The  best  British  examples  of  fresh  Tertiary  rhyolites  are 
found  in  Antrim.  Prof.  Cole11  has  described  lithoidal  varieties 

1  Fouque  and  L6vy,  pi.  xvn,  fig.  1. 

2  On  obsidians  and  spherulitic  rhyolites  from  the  Yellowstone  Park 
see  also  Rutley,  Q.  J.  G.  S.  (1881)  xxxvii,  391-396,  pi.  xx. 

3  Micro.  Petrogr.  Fortieth  Parallel  (1876),  163-205,  pi.  vi-ix. 

4  Monog.  xx  U.  S.  Geol.  Sur.  (1893)  374-380,  pi.  vm  ;  and  in  Diller, 
160,  161. 

5  Bull.  No.  66,  U.  S.  Geol.  Sur.  (1890)  10,  11. 

6  Cross,  Proc.  Colo.  Sci.  Soc.  1887,  229-233. 

7  Tower  and  Smith,  l$th  Ann.  Rep.  U.  S.  Geol.  Sur.  part  m  (1899), 
633. 

8  Diller,  Bull.  No.  148,  U.  S.  Geol.  Sur.  (1897)  192. 

9  Geol.  Black  Hills  (1880),  486-488,  etc.,  pi.  i,  figs.  1,  2. 

10  Ann.  N.  Y.  Acad.  Sci.  (1899)  xii,  284-286. 

11  Sci.  Trans.  Roy.  Dull.  Soc.  (1896)  vi,  77-118,  pi.  iv. 


166  BRITISH   RHYOL1TES. 

from  Templepatrick,  Cloughwater,  and  other  places,  and  a 
good  obsidian  is  found  at  Sandy  Braes.  In  this  last  rock 
Prof.  Watts1  has  remarked  perlitic  cracks  traversing  both 
the  brown  glassy  ground-mass  and  the  quartz  phenocrysts. 
Other  rhyolites  occur  between  Dromore  and  Moira,  Co.  Down. 
In  the  Tertiary  rhyolites  of  Fionn-choire,  in  Skye,  the  ground- 
mass  is  partly  replaced  by  streaks  and  lenticles  of  quartz2. 

The  most  interesting  British  rhyolites,  however,  are  those 
belonging  to  the  Palaeozoic  and  older  volcanic  groups,  and 
these  have  doubtless  had  their  pristine  characters  modified 
in  many  instances  by  secondary  physical  and  chemical  changes. 

Mr  Allport  was  the  first  to  give  a  clear  account  of  some 
of  the  old  altered  volcanic  glasses,  and  to  compare  them  with 
fresh  Tertiary  examples.  He  described  what  seems  to  be 
a  devitrified  and  altered  spherulitic  rhyolite  of  pre-Cambrian 
age  from  Overley  Hill  or  the  Lea  Rock  near  Wellington, 
Shropshire3.  A  few  phenocrysts  occur,  but  the  bulk  of  the 
rock  has  been  a  glass  enclosing  numerous  bands  of  spherulites. 
The  glass  is  now  devitrified,  but  perlitic  cracks,  marked  by 
secondary  products,  are  still  evident.  The  spherulites  too 
are  for  the  most  part  much  altered  and  stained  red  by  iron- 
oxide. 

The  Ordovician  rhyolites  of  Caernarvonshire4  are  charac- 
terized by  the  general  paucity  of  any  phenocrysts,  and  especially 
of  those  of  quartz5.  Among  the  scattered  felspar-crystals,  a 
member  of  the  albite-oligoclase  series  predominates  over  ortho- 
clase.  Almost  the  only  ferro-magnesian  constituent  is  a  little 
colourless  augite,  and  even  this  is  commonly  wanting,  though 
a  pale  green  decomposition-product  may  perhaps  represent  it. 
In  all  these  features  the  rocks  closely  resemble  the  Tertiary 
and  Recent  rhyolites  of  Iceland,  and  probably  the  older  rocks 

1  Q.  J.  G.  8.  (1894)  1,  367-375,  pi.  xvin. 

2  Summary  of  Progress  Geol.  Sur.  for  1897,  132-134. 

3  Q.  J.  G.  S.  (1877)  xxxiii,  449-460 ;  Teall,  pi.  xxxiv,  figs.  1,  2. 

4  Bala  Vole.  Ser.  Caern.  (1889)  18-23.     See  also  Bonney,  Q.  J.  G.  S. 
(1882)  xxxviii,  289-296,  pi.  x  ;  Eutley,  ibid.  (1881)  xxxvii,  pi.  xxi,  and 
Mem.  Geol.  Sur.,  Pels.  Lavas  (1885),  pi.  u-iv. 

5  This  is  true  more  especially  of  central  and  eastern  Caernarvonshire. 
The  rhyolites  of  the  Lleyn  peninsula,  many  of  which  are  intrusive,  are 
richer  in  phenocrysts,  including  quartz. 


BRITISH    RHYOLITES.  167 

liave  once  been  largely  glassy,  as  the  younger  are  now.  The 
usual  texture  of  these  old  lavas  is  cryptocrystalline  to  micro- 
crystalline,  sometimes  shewing  fluxion  and  banding,  and  oc- 
casionally good  perlitic  cracks.  The  vesicular  structure  is 
not  very  frequent.  In  some  types  the  ground  is  partly  micro- 
poecilitic,  minute  felspar  prisms  being  enclosed  in  quartz 
(Penmaenbach,  etc.).  Any  approach  to  a  microspherulitic 
structure  of  a  perfect  type  is  uncommon,  but  large  isolated 
spherulites  are  abundant  in  many  localities,  and  shew  the 
various  secondary  alterations,  concentric  shell-structure,  silici- 
fication,  etc.,  to  which  they  are  always  prone1.  The  siliceous 
and  other  nodules  which  thus  arise  may  reach  several  inches 
in  diameter.  Some  of  them  probably  represent  true  litho- 
physes2. 

Various  types  of  rhyolites,  including  some  with  micro- 
pegmatite  phenocrysts  (fig.  38,  7/),  occur  in  the  Ordovician 
of  Fishguard  in  Pembrokeshire3 ;  spherulitic  and  other  varieties 
on  Skomer  Island4 ;  and  imperfectly  spherulitic  types  in  the 
Precelli  Hills5.  Silicified  rhyolites  occur  at  Trefgarn  and 
Roche  Castle. 

At  Malvern  (New  Reservoir)  occur  cryptocrystalline  (per- 
haps de vitrified)  rhyolites,  sometimes  enclosing  scattered  pheno- 
crysts of  oligoclase.  Narrow  veins  are  occupied  in  some  cases 
by  infiltrations  of  calcite,  in  others  by  a  clear  mosaic  of  quartz, 
orthoclase,  and  plagioclase  of  secondary  formation.  Mr 
Parkinson6  has  described  and  figured  ancient  rhyolites  with 
nodular  structures  representing  altered  lithophyses  from 
Wrockwardine  and  Pontesford  Hill  in  Shropshire,  as  well 
as  from  Boulay  Bay  in  Jersey. 

Nodular  structures,  often  more  or  less  completely  replaced 
by  quartz,  are  seen  also  in  Westmorland  (Great  Yarlside).  The 

1  Bala  Vole.  Ser.  Caern.  35-39  ;  Cole,  Q.  J.  G.  S.  (1885)  xli,  162-168, 
pi.  iv,  and  (1886)  xlii,  185-189,  pi.  ix ;   Miss  Kaisin,  ibid.   (1889)  xlv, 
247-269. 

2  Cole,  Q.  J.  G.  S.  (1892)  xlviii,  443-445,  with  references. 

3  Heed,  Q.  J.  G.  S.  (1895)  li,  162,  pi.  vi,  figs.  3-5. 

4  Rutley,  Q.  J.  G.  S.  (1881)  xxxvii,  409-412 ;  Howard  and  Small, 
Trans.  Cardiff  Nat.  Soc.  (1897)  xxviii,  part  i,  with  plate. 

5  Parkinson,  Q.  J.  G.  S.  (1897)  liii,  465-476,  pi.  xxxvi. 

6  Ibid.  (1901)  Ivii,  218-223,  pi.  vm. 


168  OLDER   AMERICAN   RHYOLITES. 

old  rhyolites  here  resemble  in  many  respects  those  of  like  age 
in  Caernarvonshire,  but  certain  flows  shew  a  very  perfect 
microspherulitic  structure.  This  is  well  seen  in  Long  Sleddale1 
and  near  Great  Yarlside  (fig.  37).  From  Dufton  Pike,  in 
Edenside,  Mr  Rutley8  has  described  and  figured  rhyolites 
with  a  tufaceous  structure  ;  and  others  from  the  same  district 
shew  flow  brecciation  or  enclose  foreign  fragments3. 

Various  acid  lavas  occur  in  the  Ordovician  of  Ireland. 
Some  from  Raheen  and  other  places  in  Co.  Waterford  shew 
perlitic  and  microspherulitic  structures4. 

Ancient  acid  lavas  of  Palaeozoic  and  pre-Pala3ozoic  ages 
occupy  large  tracts  in  the  east  of  Canada  and  the  United 
States.  In  spite  of  alteration  they  have  preserved  many 
relics  of  original  characteristic  structures5.  This  is  well 
illustrated  by  examples  from  South  Mountain6  (Penna.), 
which  include  micropcecilitic,  spherulitic,  Kthophysal,  brec- 
ciated,  and  other  varieties.  Ancient  devitrified  obsidians 
and  rhyolites,  some  spherulitic,  have  been  described  from 
Vinal  Haven7  and  North  Haven8  in  Maine,  from  near  St  John, 
New  Brunswick9,  from  the  Michigamme  district  in  Michigan10, 
etc. 

Although  we  have  not  made  a  distinct  subfamily  of  soda- 
rhyolites,  it  may  be  remarked  that  there  are  among  the  acid 
lavas  some  characterized  by  anorthoclase  felspar  and  even 
soda-bearing  pyroxene  or  amphibole.  Some  of  the  ceratophyres 
and  quartz-ceratophyres  of  certain  authors  belong  here.  One 

1  Rutley,  Q.  J.  G.  S.  (1884)  xl,  pi.  xvm,  fig.  6,  and  Mem.  Geol.  Sur., 
Pels.  Lavas  (1885),  pi.  n,  figs.  1,  2  ;  Teall,  pi.  xxxvm  [1921]. 

Q.  J.  G.  S.  (1901)  Ivii,  31-37,  pi.  i. 

Ibid.  (1891)  xlvii,  518,  519. 

Reed,  Q.  J.  G.  S.  (1899)  Iv,  763-766. 

G.  H.  Williams,  Journ.  Geol.  (1894)  ii,  1-31. 

G.  H.  Williams,  A.  J.  S.  (1892)  xliv,  482-496  ;  F.  Bascom,  Journ. 
Geol.  (1893)  i,  813-832,  and  Bull.  136  U.  S.  G.  S.  (1896)  with  plates.  See 
also  Diller,  343-349,  pi.  XLIII,  XLIV. 

7  G.  H.  Williams,  Journ.  Geol.  (1894)  ii,  23 ;  G.  O.  Smith,  Joh.  Hopk. 
Univ.  Circ,.  No.  121  (1895),  and  Geol.  of  Fox  Is.,  Me.  (1896)  46-51,  pi.  i, 
figs.  5,  6. 

8  Bayley,  Bull.  Geol.  Soc.  Amer.  (1894)  vi,  474. 

»  Matthew,  Trans.  N.  Y.  Acad.  Sci.  (1895)  xiv,  197-200,  pi.  xn,  xm. 
10  Clements,  Journ.  Geol.  (1895)  iii,  811-817. 


PANTELLARITES.  169 

from  Marblehead  Neck,  Mass.,  has  phenocrysts  of  anortho- 
clase1.  Another  from  Baraboo  Bluffs,  Wis.,  is  also  rich  in 
soda2.  An  example  from  Berkeley,  Cal.,  ranges  from  a 
porphyritic  variety  with  microcrystalline  ground-mass  to  a 
pure  glass,  but  is  usually  microspherulitic3. 

An  regirine-bearing  rhyolite  is  described  by  Bertolio  from 
Sardinia  (Comende  type).  More  remarkable  are  the  rhyolites 
of  Pantellaria,  an  outlying  island  of  the  Lipari  group,  a 
peculiar  type  rich  in  soda  and  iron  (pantellarite).  The  pheno- 
crysts are  of  anorthoclase  and  soda-sanidine,  a  green  pleochroic 
augite,  and  the  deep-brown,  intensely  pleochroic  cossyrite. 
The  ground-mass  varies  from  almost  holocrystalline  to  almost 
wholly  vitreous,  a  prevalent  variety  being  a  glass  crowded 
with  microlites  of  the  above-mentioned  minerals.  From  the 
Vieja  Mts  in  Texas  Lord4  describes  a  quartz-pantellarite 
with  phenocrysts  of  anorthoclase,  augite,  and  quartz  in  a 
ground-mass  of  segirine-augite,  a  brown  hornblende  (probably 
barkevicite),  orthoclase,  and  quartz. 

1  Sears,  Bull   Mm.   Comp.   Zool.   Harv.   (1890)  xvi,    162-172.     Cf. 
Washington,  Joum.  Geol.  (1899)  vii,  290-292. 

2  Weidman,  Bull.  Univ.  Wis.  (1895),  Sci.  Ser.  i ;  and  in  Diller,  164- 
169,  pi.  xxvni. 

3  Palache,  Bull.  Dep.  Geol.  Univ.  Cal.  (1893)  i,  61-72. 

4  Bull.  US  U.  S.  G.  S.  (1897)  96. 


CHAPTER  XII. 

TKACHYTES  AND  PHONOLITES. 

THE  trachytes  are  lavas  which,  with  a  lower  percentage 
of  silica  than  the  rhyolites,  have  as  much  or  more  of  the 
alkalies.  Consequently  the  typical  trachytes  consist  essentially 
of  alkali-felspars  with  a  relatively  small  amount  of  coloured 
minerals  and  without  free  quartz.  The  name  trachyte  (given 
by  Haiiy  to  denote  the  rough  aspect  of  the  rocks  in  hand 
specimens)  is  used  in  the  older  literature  to  cover  all  the 
more  acid  half  of  the  volcanic  rocks.  From  it  have  been 
separated  off,  on  the  one  hand,  the  rhyolites  of  modern  no- 
menclature and,  on  the  other,  some  hornblende-  and  mica- 
andesites,  etc. 

With  the  trachytes  we  shall  treat  some  lavas  of  more 
peculiar  constitution,  in  which  a  greater  richness  in  alkalies 
has  given  rise  to  the  formation  of  felspathoids  as  well  as 
alkali-felspars  :  these  are  the  phonolites  and  leucitophyres. 
The  name  phonolite  (a  translation  of  'clinkstone,'  from  the 
supposed  sonorous  quality  of  the  rock  when  struck)  seems 
to  have  been  in  general  use  before  the  presence  of  microscopic 
nepheline  in  the  rock  was  demonstrated,  giving  a  character 
of  precision  to  the  definition.  The  original  leucitophyres  (of 
Coquand)  were  apparently  any  rocks  with  conspicuous  crystals 
of  leucite,  but  the  name  is  now  generally  restricted  to  the 
types  containing  an  alkali -felspar  (sanidine)  as  an  essential 
constituent.  The  leucitophyres  are  a  type  of  extremely 
restricted  distribution,  and  the  unstable  nature  of  the  cha- 
racteristic mineral  must  make  such  rocks  difficult  to  detect 


MINERALS   OF   TRACHYTES   AND   PHONOLITES.        171 

among  the   older  lavas,   a  remark   applicable   also  in  some 
degree  to  the  phonolites. 

Constituent  minerals.  Felspars  rich  in  potash  or  soda 
are  by  far  the  most  abundant  minerals  in  the  rocks  here 
considered.  They  occur  both  as  phenocrysts  and  as  the  chief 
element  in  the  ground-mass.  The  most  prominent  is  usually 
orthoclase  of  the  sanidine  variety,  often  shewing  a  rough  ortho- 
pinacoidal  cleavage1.  In  phenocrysts  it  has  either  a  tabular 
or  a  columnar  habit,  and  both  may  occur  in  the  same  rock. 
Carlsbad  twinning  is  frequent2,  and  in  the  larger  crystals  may 
shew  the  broken  divisional  line  due  to  interpenetration.  Some 
degree  of  zonary  banding  is  sometimes  found.  The  plagioclase 
felspar  which  occurs  in  many  trachytes  is  usually  oligoclase, 
but  in  more  basic  rocks  we  may  find  varieties  richer  in  lime 
instead.  The  phenocrysts  often  shew  carlsbad-  as  well  as 
albite-twinning ;  zonary  banding  is  not  uncommon ;  and  parallel 
intergrowth  with  sanidine  may  be  noted3  (fig.  39,  #). 

In  the  true  trachytes  the  most  common  ferro-magnesian 
element  is  perhaps  brown  biotite,  in  hexagonal  flakes  almost 
always  affected  by  corrosion  by  tbe  enclosing  magma  ('re- 
sorption').  This  is  shewn  by  a  certain  degree  of  rounding 
and  the  formation  of  a  dark  or  opaque  border,  or  even  the 
total  destruction  of  the  flake,  the  resulting  products  being 
especially  magnetite  and  sometimes  greenish  augite  in  minute 
granules.  The  frequent  preservation  of  the  original  crystal- 
forms  proves  that  the  process  is  not  one  of  fusion  and  re- 
crystallization,  but  rather  pseudomorphism  depending  on 
changed  physical  conditions  and  chemical  reactions  with  the 
fluid  magma4.  Brown  hornblende  is  a  less  frequent  constituent, 
in  idiomorphic  crystals  with  similar  resorption-phenomena. 
The  augite,  which  is  scarcely  less  common  than  biotite  as 
a  constituent  of  trachytes,  never  shews  this  feature.  It  is 
usually  pale  green  in  thin  slices.  In  the  phonolites  and 
leucitophyres  the  crystal  often  shews  a  deeper  tint  at  the 
margin,  and  is  almost  always  sensibly  pleochroic  (segirine- 

1  Cohen  (3),  pi.  XLVII,  fig.  3. 

2  Ibid.  pi.  xxiv,  fig.  1. 

3  Ibid.  pi.  xxxii,  fig.  3. 

4  Washington,  Journ.  Geol.  (1896)  iv,  257-282. 


172  FELSPATHOID   MINERALS   OF   PHONOLITES. 

augite),  a  character  less  common  in  the  trachytes.  The  soda- 
pyroxene,  wgirine,  is  characteristic  of  many  phonolites  and 
leucitophyres,  but  only  occasionally  present  in  the  trachytes. 
It  is  green  and  pleochroic,  with  a  much  lower  extinction-angle 
than  the  augites  (5°  or  less  in  longitudinal  sections).  It  some- 
times grows  round  a  kernel  of  augite  with  parallel  orientation. 
The  rhombic  pyroxene  of  certain  trachytes  is  always  of  a  deeply 
coloured  and  vividly  pleochroic  variety  (hypersthene  or  ambly- 
stegite),  giving  red-brown,  yellow-brown,  and  green  colours  for 
the  several  principal  directions  of  absorption. 

The  nepheline  of  the  phonolites  and  leucitophyres  occurs 
in  minute  crystals  in  the  ground-mass,  having  the  form  of 
a  short  hexagonal  prism  with  basal  planes,  and  giving  squarish 
or  hexagonal  sections  (fig.  40,  A).  Owing  to  the  small  size  of 
the  crystals  and  the  optical  properties  of  the  mineral,  it  is 
liable  to  be  overlooked.  Its  decomposition  gives  rise  to 
various  soda-zeolites,  which  occur  in  nests  and  veins  in  many 
phonolites.  The  leucite  of  the  leucitophyres  is  always  idio- 
morphic,  giving  characteristic  octagonal  and  rounded  sections 
(fig.  40,  B).  Twin-lamellation  is  very  frequent  in  the 
phenocrysts1,  but  the  smaller  crystals  which  may  occur  often 
behave  almost  as  if  isotropic.  The  leucite  may  enclose 
needles  of  augite  and  crystals  of  the  earlier-formed  minerals, 
but  not  of  felspar.  Minerals  of  the  sodalite  group  are  found 
in  certain  trachytes  and  constantly  in  the  phonolites  and 
leucitophyres.  They  are  almost  always  in  idiomorphic  dode- 
cahedra.  The  sodalite  is  clear  when  fresh,  but  often  turbid 
from  alteration :  zonary  structure  is  frequent.  The  blue 
hauyne  is  less  often  met  with,  but  nosean  may  be  very 
plentiful,  usually  forming  crystals  of  some  size,  and  always 
shewing  more  or  less  plainly  its  characteristic  structure  and 
border2  (fig.  40,  B).  The  sodalite-minerals  give  rise  by 
alteration  to  natrolite  and  other  zeolites. 

Iron-ores  (magnetite)  occur  but  sparingly  in  these  rocks. 
Yellowish  sphene  in  good  crystals  is  highly  characteristic  ;  and 
apatite  is  common  in  colourless  needles  or  sometimes  in  rather 
stouter  prisms  with  violet  dichroism.  The  trachytes  often 
contain  a  little  zircon  in  minute  prisms. 

1  Cohen  (3),  pi.  xxvni,  fig.  4.          2  Teall,  pi.  XLI,  fig.  1;  XLVII,  fig.  4. 


ACCESSORY    MINERALS   OF   TRACHYTES,   ETC.  173 

Among  less  common  minerals  may  be  mentioned  the  tri- 

rite  of  certain  trachytes,  in  aggregates  of  minute  flakes ; 
'ne,  as  a  rare  constituent  except  in  certain  basic  trachytes  ; 
and  melanite  garnet,  which  is  found  in  some  of  the  leucito- 
phyres  and  in  certain  phonolites  as  brown  isotropic  crystals 
belonging  to  an  early  stage  of  consolidation,  sometimes  shew- 
ing marked  zonary  banding. 

As  secondary  products  in  trachytic  (as  also  in  andesitic) 
rocks,  opal  and  other  forms  of  soluble  silica  are  not  uncommon. 
Normally  isotropic,  these  substances  sometimes  shew  double 
refraction  as  a  consequence  of  strain,  usually  about  centres, 
so  as  to  imitate  a  spherulitic  structure.  Opal  sometimes  en- 
closes little  flakes  or  aggregates  of  tridymite,  or  is  coloured 
red  by  included  scales  of  haematite.  It  occurs  in  the  form  of 
veins  and  irregular  knots  or  patches.  Aggregates  of  minute 
scales  of  tridymite  are  common  in  certain  trachytic  lavas, 
such  as  the  'domites'  of  Auvergne1. 

Ground-mass.  In  contrast  with  the  rhy elites,  the  rocks 
under  consideration  have  few  glassy  representatives,  and  the 
ground-mass  is  frequently  holocrystalline,  or  at  least  with  no 
sensible  amount  of  glassy  residue.  This  is  especially  true  of 
the  typical  trachytes,  which,  with  a  chemical  composition  not 
very  different  from  that  of  a  mixture  of  felspars,  have  a  strong 
tendency  to  crystallize  bodily.  Fluxional  phenomena  are  not 
conspicuous,  and  the  characteristic  banding  of  the  rhyolites 
is  here  wanting.  Vesicular  structure  is  rare,  and  perlitic 
cracks  are  not  formed ;  but,  in  consequence  of  the  crystalline 
nature  of  the  ground,  with  a  tendency  to  idiomorphism  in  its 
elements,  a  miarolitic  or  drusy  structure  may  be  met  with. 
Any  structure  comparable  with  the  spherulitic  is  uncommon, 
though  a  rough  radial  grouping  of  felspar  prisms  is  sometimes 
observable. 

Excepting  the  nepheline  of  the  phonolites,  non-felspathic 
constituents  play  in  most  cases  a  small  part  in  the  ground- 
mass  of  the  rocks  here  considered.  The  ground  consists,  in 
the  trachytes  proper,  essentially  of  minute  felspars,  which  may, 
however,  vary  somewhat  in  habit.  Most  commonly  they  are 

1  Cohen  (3),  pi.  xxxvn,  fig.  1. 


174         STRUCTURES  OF  TRACHYTES,  ETC. 

'lath-shaped'  microlites,  with  some  degree  of  parallel  dis- 
position in  consequence  of  flow,  and  this  type  of  ground  is  so 
characteristic  of  these  rocks  that  it  is  often  styled  the  trachytic1. 
On  the  other  hand  the  minute  felspars  may  have  a  shorter  and 
stouter  shape,  recalling  some  of  the  rocks  grouped  above  under 
the  porphyries,  and  this  structure  is  accordingly  designated 
by  Rosenbusch  the  orthophyric2  (fig.  39). 

Phonolites  poor  in  nepheline  do  not  differ  essentially  as 
regards  structures  from  the  trachytes ;  but  when  the  character- 
istic mineral  is  plentiful,  forming  very  numerous  minute 
crystals  in  the  ground-mass,  the  general  aspect  of  the  latter 
is  somewhat  altered.  The  leucitophyres  shew  in  their  very 
variable  structures  further  departures  from  the  trachyte  type, 
and  the  porphyritic  character  is  sometimes  lost;  but  all  the 
rocks  included  in  the  present  chapter  resemble  one  another  in 
being  normally  holocrystalline. 

Leading  types.  Among  the  best  known  foreign  trachytes 
are  those  of  the  Siebengebirge  (Drachenfels  type).  Here  a 
ground-mass  of  lath-shaped  felspar  microlites,  with  typical 
trachytic  structure,  encloses  crystals  of  sanidine  and  oligoclase. 
The  former  are  frequently  of  large  size,  and  may  shew  carlsbad 
twinning.  Biotite  and  magnetite  occur  sparingly.  The  rock 
of  Perlenhardt  in  the  same  district  exemplifies  what  llosen- 
busch  styles  the  orthophyric  type  of  ground-mass.  A  little 
green  augite  accompanies  the  biotite,  sphene  is  common,  and 
sodalite  occurs  in  crystals  or  in  crystalline  patches.  In 
America  a  trachyte  of  the  Drachenfels  type  has  been  described 
by  Cross3  from  the  neighbourhood  of  Rosita  in  Colorado,  and 
similar  rocks  are  found  in  the  Black  Hills  of  Dakota4. 

Trachytes  from  Solfatara  and  Mte  Olibanp5  resemble 
generally  the  Perlenhardt  rock.  Numerous  augite-trachytes 
are  found  in  the  neighbourhood  of  Naples  and  the  Phlegraean 
Fields6.  In  Britain  some  very  fresh  augite -bearing  trachytes 

1  Berwerth,  Lief.  i.  (augite-trachyte,  Naples). 

2  Ibid.  Lief.  n.  (domite,  Auvergne). 

3  Proc.  Colo.  Sci.  Soc.  1887,  234  ;  and  in  Diller,  181,  182. 

4  Caswell.  Geol.  Black  Hills  (1880),  488-492,  etc.,  pi.  11. 

5  Cohen  (3),  pi.  xv,  fig.  2. 

6  Berwerth,  Lief.  i. 


BRITISH   TRACHYTES.  175 

occur  as  lava-flows  and  volcanic  necks  of  Lower  Carboniferous 
age  in  the  Garlton  Hills,  Haddingtonshire1.  These  rocks 
consist  of  alkali-felspars  with  more  or  less  of  a  bright  to  pale 

freen  pleochroic  augite,  doubtless  a  soda-bearing  variety, 
pecimens  from  Peppercraig  (fig.  39)  shew  phenocrysts  of 
sanidine,  sometimes  with  intergrowths  of  oligoclase,  in  a 
holocrystalline  ground-mass.  The  latter  is  chiefly  of  sanidine 
prisms,  with  a  minor  proportion  of  striated  felspar.  Augite 
builds  imperfect  crystals  and  grains  and  numerous  smaller 
granules  ;  magnetite  occurs  sparingly  in  the  same  manner ; 
and  occasional  needles  of  apatite  are  seen.  Trachytes  of  Old 
Red  Sandstone  age  are  found  in  the  neighbourhood  of  Melrose. 
Some  of  these  contain  no  mineral  of  the  ferro-magnesian 
division.  One  from  Easter  Eildon  Hill  is  described  by 
Mr  Barron2  as  carrying  riebeckite. 


B 


FIG.  39.     AUGITE -TRACHYTE,  PEPPERCRAIG,  HABDINGTON  ;    x  20. 

A  in  natural  light,  B  between  crossed  nicols.  Large  phenocrysts  of 
felspar  are  enclosed  in  a  ground  composed  entirely  of  little  felspar  prisms 
and  granules  of  augite  [1980].  The  structure  is  orthophyric. 

1  Hatch,   Trans.  Roy.  Soc.  Edin.  (1892)    xxxvii,  115-126;    see   also 
Geikie,  ibid.  (1879)  xxix,  pi.  xn,  figs.  1,  2.     The  Carboniferous  trachytes 
described  by  McMahon  from  Dartmoor  seem  to  be  much  altered  and 
their  characters  obscured ;  Q.  J.  O.  S.  (1894)  1,  345,  346. 

2  G.  M.  1896,  376. 


176  MORE   BASIC   TRACHYTES. 

In  those  trachytes  which  in  some  respects  approach  the 
andesites,  the  coloured  constituents,  especially  pyroxene,  be- 
come relatively  abundant,  and  plagioclase  begins  to  pre- 
dominate over  orthoclase  among  the  phenocrysts.  A  type 
from  Mte  Amiata  in  Tuscany  and  M.  Dore  in  Auvergne  con- 
tains a  vividly  pleochroic  rhombic  p}Toxene  (amblystegite) 
with  subordinate  biotite.  Garnet  and  tridymite  are  accessories. 
The  ground-mass  of  these  rocks  is  of  very  variable  character, 
even  in  the  same  flow,  and  is  sometimes  largely  glassy. 
Washington1  has  proposed  the  name  vulsinite  for  a  group 
of  rocks  intermediate  between  trachyte  and  andesite.  They 
contain  a  considerable  amount  of  a  basic  plagioclase  in  addition 
to  the  alkali-felspar,  and  the  ferro-magnesian  constituent  is 
typically  augite.  In  examples  from  Bolsena  in  Italy  the 
phenocrysts  are  of  alkali-felspar,  anorthite,  augite,  and  biotite, 
and  the  ground-mass  is  of  soda-orthoclase,  augite,  etc.,  with 
trachytic  structure.  One  from  the  Viterbo  district  has 
labradorite  in  place  of  anorthite. 

A  somewhat  more  basic  type,  from  the  Mti  Cimini  in  the 
latter  district,  is  styled  citninite2.  It  has  the  same  association 
of  sanidine  with  a  basic  felspar,  but  carries  phenocrysts  of 
olivine,  as  well  as  of  augite  and  felspar.  A  well-known 
example  of  this  is  the  Arso  trachyte,  the  Ischia  lava  of  A.D. 
1302,  which  approximates  in  some  features  to  the  basalts. 
The  ground-mass  is  of  felspar  microlites  with  interstitial 
glass,  and  is  sometimes  vesicular. 

Other  trachytes  shew  an  approach  to  the  characters  of 
phonolites  in  the  abundance  of  sodalite,  the  ocurrence  of 
aegirine,  etc.  The  trachytes  of  the  Laacher  See  in  the  Eifel 
have  crystals  of  sodalite  and  haiiyne,  besides  sanidine  and 
oligoclase.  Biotite,  brown  hornblende,  segirine,  sphene,  mag- 
netite, etc.,  also  occur,  and  the  ground-mass  is  of  the  trachytic 
type.  At  the  Laach  volcano  are  found  also  ejected  blocks  of 
a  rock  named  sanidine-trachyte  or  sanidinite.  This  consists 
essentially  of  sanidine  with  subordinate  oligoclase,  sodalite, 
occasional  biotite,  etc.  Stellate  groupings  of  crystals  occur 

1  Journ.  Geol.  (1896)  iv,  547-554,  833. 

2  Journ.  Geol.  (1896)  iv,  834-838,  and  (1897)  v,  354. 


SODA-TRACHYTES,    ETC.  177 

in  both  felspars,  but  on  the  whole  the  structure  is  that  of  a 
plutonic  (syenitic)  rather  than  a  volcanic  rock. 

While  the  dominant  mineral  of  the  trachytic  lavas  is 
commonly  a  potash-felspar,  there  are  some  types  very  rich  in 
soda  ;  albite,  anorthoclase,  or  some  allied  felspar  occurring 
almost  to  the  exclusion  of  sanidine  or  orthoclase.  The  'quartz- 
less  pantellarites'  of  Pantellaria  must  be  placed  here,  and 
the  older  equivalents  of  such  types  are  to  be  sought  among 
some  of  the  rocks  which  have  been  styled  quartzless  cerato- 
phyres.  A  very  interesting  soda-felspar-rock  has  been  described 
from  Dinas  Head  on  the  north  coast  of  Cornwall1.  This  is 
possibly  to  be  regarded  as  an  ancient  lava2,  and  it  consists 
almost  wholly  of  albite.  Besides  a  compact  variety,  there  are 
others  which  are  spherulitic  and  nodular.  The  centre  of  a 
spherule  is  cryptocrystalline,  while  its  outer  portion  consists 
of  radiating  blades  of  albite.  Such  rocks  may  be  termed  old 
soda- trachytes,  corresponding  with  the  soda-rhyolites  which 
are  also  known  in  this  country.  Lavas  consisting  almost 
wholly  of  alkali-felspars  occur  at  Hamilton  Hill  and  other 
places  near  Peebles3.  Small  felspar  phenocrysts  are  embedded 
in  a  felspathic  ground-mass  of  microlitic  or  cryptocrystalline 
structure,  and  analysis  shews  that  soda-felspar  largely  pre- 
dominates. 

Purely  glassy  varieties  (trachyte-obsidians)  are  uncommon 
in  this  family.  In  the  localities  where  they  are  found,  they 
are  associated  with  trachytes  wholly  or  mainly  crystalline, 
or  even  narrow  alternating  bands  occur  of  pure  glass  and 
of  trachyte  largely  microcrystalline.  Good  examples  of  this 
occur  in  the  Peak  of  Tenerife.  It  may  be  noted  that  a  glassy 
variety  of  phonolite  also  is  found  in  the  Canaries,  usually  as 
a  slaggy  crust  on  the  surface  of  a  lava-flow.  It  is  a  brown 
or  yellow  glass  with  little  development  of  crystallites. 

Coming  now  to  the  phonolites,  we  notice  first  those  in 
which  nepheline  is  only  sparingly  present,  and  which  thus 
stand  in  close  relation  with  the  trachytes.  Such  rocks,  the 

1  Howard  Fox,  G.  M.  1895,  13-20. 

2  McMahon,  however,  regards  it  as  a  metamorphosed  sediment ;  ibid. 
257,  258. 

3  Teall,  Ann.  Rep.  Geol.  Sur.  for  1896,  40. 

H.  P.  12 


178  BRITISH   AND   OTHER   PHONOLITES. 

'trachytoid'  phonolites  of  Rosenbusch,  are  not  the  most 
characteristic  type ;  and  the  'nephelinitoid'  group,  in  which 
the  special  mineral  of  the  phonolites  is  more  abundantly 
present,  is  commoner.  Some  of  the  Saxon  phonolites  are  of 
the  trachytoid  type  (Olbersdorf,  near  Zittau).  A  good  example 
is  found  at  Traprain  Law  in  association  with  the  trachytes 
of  the  Garlton  Hills,  Haddingtonshire1,  and  is  of  interest 
as  being  of  Carboniferous  age.  It  consists  essentially  of  a 
mass  of  little  sanidine  prisms,  with  a  fluxional  arrangement, 
in  which  lie  ragged  crystals  of  a  bright  green  soda-augite. 
Small  colourless  patches  are  found  on  very  close  examination 
to  consist  of  little  crystals  of  nepheline  with  zeolitic  decom- 
position-nroducts.  A  lava  from  Middle  Eildon  Hill,  near 
Melrose,  is  also  a  phonolite  of  trachytoid  type,  and  is  remark- 
able for  having  riebeckite  instead  of  aegirine.  The  mineral 
occurs,  as  usual,  in  irregularly  shaped  patches,  moulded  on  the 
felspar.  This  rock  is  of  Upper  Old  Red  Sandstone  age2. 

Of  the  commoner  type  of  phonolite  good  examples  occur 
in  Bohemia3  (Briix,  Teplitz,  Marienberg,  etc.),  sanidine,  nephel- 
ine, and  segirine  being  the  essential  minerals.  Some  varieties 
have  conspicuous  phenocrysts  of  sanidine.  At  the  Roche 
Sanadoire4  in  Auvergne  the  porphyritic  sanidines  have  often 
a  core  of  plagioclase  in  parallel  intergrowth,  and  little  lath- 
shaped  crystals  of  plagioclase  occur  also  in  the  ground-mass. 

Another  British  phonolite — that  of  the  Wolf  Rock  off 
the  coast  of  Cornwall — is  also  a  good  example.  It  belongs 
to  the  nosean-phonolites  of  some  authors,  that  mineral  being 
found  plentifully  in  it,  in  addition  to  nepheline.  The  nosean 
occurs  chiefly  as  phenocrysts  with  a  dark  interior  and  clear 
border5.  Sanidine  is  also  found  as  phenocrysts.  The  general 
mass  of  the  rock  consists  of  lath-shaped  sanidine  crystals, 
more  or  less  idiomorphic  crystals  of  nepheline,  and  little,  dirty 
green  microlites  of  segirine.  Iron-ores  are  scarcely  represented, 
and  there  is  little  or  no  residual  glass. 

1  Hatch,  G.  M.  1892,  149 ;  Trans.  Roy.  Soc.  Edin.  (1892)  xxxvii,  124. 

2  Barren,  G.  M.  1896,  373-375. 

3  For  chromolithograph  see  Berwerth,  Lief,  iv  (Briix). 

4  Fouque"  and  Levy,  pi.  XLVII,  fig.  1  ;  cf.  fig.  2  and  pi.  XLVI. 
c  Teall,  pi.  XLVII,  fig.  4  (misplaced  5  in  key-plate). 


AMERICAN    PHONOLITES. 


179 


Phonolites  are  only  sparingly  represented  among  the  varied 
volcanic  rocks  of  the  United  States.  One  from  El  Paso 
County,  Colorado1,  is  essentially  a  finely  granular  aggregate 
of  sanidine,  nepheline,  and  hornblende,  with  phenocrysts  ^  of 
the  two  former  minerals.  A  similar  rock,  with  the  addition 
of  a  little  nosean,  is  known  from  Black  Butte  in  the  Black 


B 


FIG.  40;   x  20. 

A.  Phonolite,  Black  Hills,  S.  Dakota.     Phenocrysts  of  soda-sanidine 
and  aegirine  in  a  ground-mass  of  nepheline  and  sanidine  [3072]. 

B.  Leucitophyre,   Burgberg,    near    Kieden,   Eifel.     Phenocrysts  of 
green  asgirine-augite,  dark-bordered  nosean,  and  clear  leucite  in  aground- 
mass  of  aegirine-augite,  nepheline,  and  sanidine  [G.  120]. 

Hills  of  Dakota2.  The  felspar  phenocrysts  are  of  soda-ortho- 
clase  or  anorthoclase3.  Phonolites  occur  as  volcanic  dykes  and 
larger  masses  in  the  Cripple  Creek  mining  district,  Colorado4. 
They  are  rich  in  alkali-felspars,  and  contain  phenocrysts  of 
soda-sanidine  or  anorthoclase.  Nepheline  occurs  with  variable 

1  Cross,  Proc.  Colo.  Sci.  Soc.  1887,  167,  168. 

2  Caswell,  Geol  Black  Hills,  U.  S.  G.  and  G.  Stir.  Rocky  Mts  (1880), 
503-505,  pi.  i,  figs.  3,  4  ;  Cross  in  Diller,  191-193. 

3  Pirsson,  A.  J.  S.  (1894)  xlvii,  341-346. 

4  Cross,  16t/i  Ann.  Rep.  U.  S.  Geol.  Sur.  part  n  (1895),  25-36. 

12—2 


180  LEUCITOPHYRES. 

habit,  sometimes  building  small  phenocrysts,  while  porphyritic 
nosean  and  minute  crystals  of  sodalite  are  also  found.  jEgirine 
and  segirine-augite  are  the  coloured  minerals,  or  in  certain  cases 
a  blue  amphibole,  and  among  the  accessory  minerals  is  analcime, 
believed  to  be  of  primary  origin.  Osann's  rocks  from  western 
Texas  (Apache  type)  are  rich  in  hornblende,  including  a  blue 
variety,  and  the  felspars  shew  microperthitic  intergrowths. 
Other  phonolites  from  this  district  are'  very  rich  in  a3girine 
and  nepheline1. 

A  remarkable  phonolitic  rock  is  recorded  from  Kosciusko 
in  New  South  Wales2.  Nepheline  is  extremely  abundant, 
and  occurs  in  microporphyritic  idiomorphic  crystals.  -ZEgirine 
is  also  abundant,  but  there  are  no  phenocrysts  of  sanidine. 
There  is  a  small  amount  of  glassy  base,  through  which  are 
scattered  microlites  of  felspar.  Phonolites  of  more  than  one 
variety  occur  in  the  neighbourhood  of  Dunedin,  New  Zealand3. 

The  leucitopkyres*  are  a  very  small  group  of  rocks,  known 
only  from  a  few  districts,  and  best  developed  in  the  late  Tertiary 
lavas  of  the  Eifel.  The  leucite  is  often  of  two  generations, 
the  larger  crystals  being  frequently  of  irregular  shape.  It  is 
always  accompanied  by  nosean  and  sanidine  (fig.  40,  H).  The 
ferro-magnesian  mineral  is  a  green  pleochroic  augite  with 
zonary  banding  :  the  other  constituents  are  sphene,  occasionally 
biotite,  and  often  a  little  melanite.  The  structure  of  the 
rocks  is  very  variable.  In  some  there  is  a  well-defined 
ground-mass  of  minute  nepheline,  sanidine,  augite,  and  leucite, 
enclosing  phenocrysts  of  leucite  and  nosean  (Olbriick,  etc.}.  In 
other  varieties  there  is  but  little  sanidine  (Schorenberg), 
while  others  again  have  sanidine  in  large  shapeless  plates 
enclosing  the  other  constituents  instead  of  a  ground-mass 
(Perlerkopf).  Leucitophyres  shewing  some  variety  of  characters 
occur  at  several  volcanic  centres  in  Italy5. 

1  Osann,  4th  Ann.  Rep.  Geol.  Sur.  Tex.  (1892)  130,  131. 

2  Guthrie,  David,  and  Woolnough,  Roy.  Soc.  N.  S.  W.  (1901). 

3  Ulrich,  Tram.  Austral.  Ass.  (1891)  iii,  127-150,  pi.  v. 

4  For  figures  see  Berwerth,  Lief,  iv ;  Cohen  (3),  pi.  11,  fig.  3,  iv,  fig.  3, 
and  xvi,  figs.   1,  2  ;  Fouque  and  Levy,  pi.  XLVIII,  fig.  1  and  LI,  fig.  1  ; 
Teall,  pi.  XLI,  fig.  2  and  XLVII,  fig.  4. 

5  Washington,  Journ.   Geol.  (1896)  iv,   559-561    (Bolsena),   840-845 
(Viterbo)  ;  (1897)  v,  43  (L.  Bracciano),  and  248,  249  (Kocca  Monfina). 


CHAPTER  XIII. 

ANDESITES. 

IN  this  family  we  include  all  the  lavas  of  *  intermediate ' 
composition  not  embraced  in  the  preceding  chapter.  The 
name  andesite,  first  used  by  von  Buch  and  derived  from  the 
prevalence  of  such  rocks  in  the  Andes1,  is  roughly  equivalent 
to  Abich's  '  trachydolerite,'  implying  the  intermediate  position 
of  these  lavas  between  the  acid  ones  (trachytes  of  older  writers) 
and  the  basic  (dolerites).  The  characteristic  minerals  are  a 
soda-lime-felspar  and  one  or  more  ferro-magnesian  minerals. 
The  alkali-felspars  and  quartz  of  the  acid  rocks  are  typically 
absent,  as  are  also  the  lime-felspar  and  olivine  of  the  basic 
rocks.  The  andesites  are  distinguished,  according  to  the 
dominant  ferro-magnesian  constituent,  as  hornblende-,  mica-, 
augite-,  and  hypersthene-andesites.  Further  there  is  usually 
recognized  a  quartz-bearing  and  more  acid  division,  known  as 
dacites  or  quartz-andesites.  Having  regard  to  true  lavas, 
these  quartz-bearing  andesites  seem  to  be  of  somewhat  limited 
distribution  :  many  of  the  rocks  described  as  '  dacites '  are  of 
hypabyssal  types,  and  belong  to  the  less  acid  quartz-porphyries. 


Those  petrologists  who  restrict  the  name  andesite  to  rocks 
of  late  geological  age,  apply  to  their  pre-Tertiary  equivalents 
the  name  'porphyriteV  tinder  the  same  title  they  include 


1  It  should  be  remarked,  however,  that  the  early  usage  of  the  word 
was  different,   or  at  least  wider,  including  rocks  of  plutonic  habitus 
(quartz-diorites). 

2  Many  also  of  the  rocks  designated  '  melaphyre '  are  pyroxene-ande- 
sites,  others  being  basalts. 


182  FELSPARS  OF   ANDESITES. 

various  rocks  of  hypabyssal  types,  and  it  is  to  these  latter  that 
we  have  already  confined  the  name.  Again,  certain  English 
petrologists  have  used  the  name  '  porphyrite '  for  andesites 
which  have  undergone  some  degree  of  change  by  weathering, 
etc.,  a  distinction  which  seems  scarcely  important  enough  to 
be  recognized  in  classification  or  nomenclature. 

As  regards  the  general  affinities  of  the  family,  the  dacites 
have  features  in  common  with  the  rhyolites,  the  hornblende- 
and  mica-andesites  with  the  trachytes,  and  the  pyroxene- 
andesites  with  the  basalts,  marking  thus  the  intermediate 
position  held  among  the  volcanic  rocks  by  the  lavas  here 
considered.  As  regards  the  appropriateness  of  the  name,  it 
is  remarkable  that  the  lavas  of  the  great  volcanic  belt  of  the 
Andes  belong,  in  so  far  as  they  are  known,  almost  exclusively 
to  this  family1. 

Phenocrysts.  Soda-lime-felspars  are  the  most  abundant 
elements  porphyritically  developed  in  these  rocks.  They  in- 
clude members  varying  from  oligoclase  to  anorthite,  but 
andesine  and  labradorite  are  the  most  common.  As  a  rule, 
the  more  acid  plagioclase  belongs  to  the  hornblende-  and 
mica-andesites  and  dacites,  the  more  basic  to  the  pyroxene- 
andesites2.  The  crystals,  however,  are  often  strongly  zoned3 
(fig.  41,  B\  shewing  a  change  from  a  more  basic  variety  in 
the  centre  to  a  more  acid  at  the  margin.  They  are  idiomorphic 
and  of  tabular  habit.  With  albite-lamellation  is  frequently 
associated  twinning  on  the  pericliue  or  on  the  carlsbad  law. 
The  commonest  inclusions  are  glass- cavities4,  either  as  '  nega- 
tive crystals,'  or  rounded  :  sometimes  large  irregular  cavities 
occupy  much  of  the  bulk  of  a  crystal5.  The  decomposition- 
products  of  the  felspars  are  calcite,  finely  divided  kaolin  or 
mica,  epidote,  quartz,  etc.  When  an  alkali-felspar  occurs  as 


1  Cf.  Iddings,  Jonrn.  Geol.  (1893)  i,  164-175. 

2  French  petrologists  recognize  '  andesites  '  and  '  labradorites '  as  dis- 
tinct groups,  characterized  by  andesine  and  labrador- felspar  respectively, 
but  this  is  with  reference  to  the  ground-mass. 

a  Iddings,  Monog.  xx  U.  S.  Geol.  Sur.  (1893)  pi.  v,  figs.  1,3,4;  vi, 
fig.  2. 

4  See  Zirkel,  Micro.  Petr.  Fortieth  Parall.  pi.  v,  fig.  3 ;  xi,  fig.  2. 

5  Cohen  (3),  pi.  vin,  fig.  1. 


MINERALS   OF   ANDESITES.  183 

an  accessory,  it  has  the  same  characters  as  in  the  rhyolites 
and  trachytes. 

The  hornblende  of  andesites  is  in  idiomorphic  prisms,  often 
twinned.  It  is  usually  a  brown  pleochroic  variety  with  quite 
low  extinction-angle,  but  green  hornblende  also  occurs.  The 
mica  is  a  brown,  strongly  pleochroic  biotite,  with  extinction 
sometimes  sufficiently  oblique  to  shew  lamellar  twinning 
parallel  to  the  base.  Both  hornblende  and  biotite  shew  the 
same  resorption-phenomena1  as  in  the  trachytes.  It  is  possible 
that  some  part  of  the  finely  divided  magnetite  and  granular 
augite  in  the  ground-mass  of  certain  andesites  comes  from 
the  breaking  up  of  hornblende  altered  in  this  way2.  By  de- 
composition of  the  ordinary  kind  the  hornblende  and  mica  of 
andesites  give  rise  to  chlorite,  magnetite,  carbonates,  etc. 

The  augite  is  in  well-shaped  crystals,  light  green  and 
usually  without  sensible  pleochroism.  T  win-lam  ellation  is 
common.  Alteration  may  give  rise  to  chlorite,  epidote,  calcite, 
etc.  The  rhombic  pyroxene  in  the  andesites  is  usually  hyper- 
sthene3,  or  at  least  a  distinctly  coloured  and  more  or  less 
pleochroic  variety.  It  builds  idiomorphic  crystals,  in  which 
the  pinacoid  faces  are  more  developed  than  the  prism ;  so 
that  the  cross-section  is  a  square  with  truncated  corners,  as 
contrasted  with  the  regular  octagon  of  augite.  In  longitudinal 
sections  the  straight  extinction  is  of  course  characteristic. 
The  rhombic  pyroxene  is  often  converted  in  the  older  rocks 
to  bastite. 

The  quartz  of  the  dacites  is  either  in  good  hexagonal 
pyramids  or  more  or  less  rounded  and  corroded,  with  inlets  of 
the  ground-mass. 

Original  iron-ores  are  usually  not  abundant :  magnetite  is 
the  only  one  commonly  found.  Needles  of  apatite  occur,  and 
in  the  more  acid  andesites  little  zircons4.  Some  of  the  more 


1  Fouque  and  Levy,  pi.  xxvm,  xxix ;  Zirkel,  Micro.  Petrogr.  Fortieth 
Parallel,  pi.  v,  tig.  2. 

2  Washington,  Journ.  Geol.  (1896)  iv,  273-278. 

3  Cross,  Bull.  No.  1  U.  S.  Geol.  Sur.  (1883) ;  A.  J.  S.  (1883)  xxv,  139; 
Teall,  G.  M.  1883,  145-148. 

4  Iddings,  Monog.  xx  U.  S.  Geol.  Sur.  (1893)  pi.  in,  figs.  15-20. 


184  STRUCTURES   OF   ANDESITES. 

basic  rocks  have  sparingly  phenocrysts  of  olivine.  As  oc- 
casional accessories  may  be  noted  tridymite1  (in  druses), 
garnet,  and  cordierite. 

Structure  of  ground-mass.  In  many  andesites  the 
only  mineral  which  occurs  distinctly  in  two  generations  is 
the  felspar.  The  felspar  of  the  ground-mass  builds  little 
4  lath- shaped '  crystals,  often  simple,  sometimes  twinned,  but 
usually  without  repetition.  It  is  probably,  as  a  rule,  of  a 
more  acid  variety  than  the  porphyritic  felspar,  andesine  or 
oligoclase  occurring  in  different  cases.  Augite  also  may  be 
present  as  a  constituent  of  the  ground-mass,  forming  very 
small  crystals  of  pale-green  tint. 

Some  of  the  hornblende-  and  mica-andesites  have  a  trach- 
ytic  type  of  ground-mass,  composed  essentially  of  very  small 
felspar-laths  with  little  or  no  glassy  base,  as  in  the  Drachenfels 
trachyte.  It  is  not  always  easy  to  ascertain  whether  any 
glass  is  present  or  not.  From  this  type,  as  from  the  others, 
there  are,  however,  transitions  to  rocks  with  a  ground-mass 
mainly  glassy. 

Less  common  is  a  *  microfelsitic '  or  cryptocrystalline 
structure.  This  is  seen  in  some  of  the  dacites.  In  some 
cases  spherulitic  structures  are  found. 

In  most  typical  andesites,  and  especially  in  the  pyroxene- 
bearing  kinds,  the  ground-mass  has  the  very  distinctive 
'felted'  character  termed  by  Rosenbusch  hyalopilitic2 .  This 
consists  of  innumerable  small  felspar-laths,  simple  or  once 
twinned,  often  with  evident  flow-structure,  and  a  residuum 
of  glassy  matter.  Vesicles  are  common,  and  their  infilling 
by  secondary  products  gives  rise  to  amygdules3.  So  charac- 
teristic is  this  type,  that  it  is  often  spoken  of  as  the  'andesitic' 
ground -mass.  When  the  little  felspars  are  closely  packed 
together,  to  the  exclusion  of  any  glassy  base,  we  have  the 
pilotaxitic  structure  of  Rosenbusch.  On  the  other  hand,  by 

1  Koto,  Q.  J.  G.  S.  (1884)  xl,  441,  444. 

2  See    chromolithograph    of    augite-andesite    ('  augite-porphyrite '), 
Berwerth,  Lief.  i. 

3  Very  many  of  the  amygdaloidal  lavas   (Ger.  Mandelstein)  belong 
here;   Cohen  (3),  pi.  LXXX,  figs.  2-4. 


DACITES.  185 

increase  in  the  proportion  of  isotropic  base,  these  andesites 
graduate  into  more  or  less  perfectly  glassy  forms.  Wholly 
glassy  types  (andesite-obsidian,  including  andesite-pumice)  are 
known  in  small  development  only,  except  in  so  far  as  they 
form  part  of  tuffs,  etc.  Some  andesitic  rocks  shew  various 
kinds  of  variolitic  structures1  comparable  with  those  seen  in 
basalts  (see  fig.  44,  A,  on  p.  198). 

Leading  types.  Among  dacites  the  best  known  are 
those  of  Tertiary  age  in  Transylvania2  and  Hungary.  They 
include  holocrystalline  examples  (Kis  Sebes,  Kpdna)  and 
others  with  cryptocrystalline  and  microspherulitic  ground- 
mass  (Schemnitz  district,  etc.),  as  well  as  those  having  the 
hyalopilitic  structure  so  common  among  the  andesites. 
Hornblende-dacite  with  microspherulitic  structure  occurs 
among  the  recent  lavas  of  Santorin  in  the  Grecian  Archi- 
pelago3. Zeolites  and  isotropic  opal  are  found  as  secondary 
products,  or  in  other  cases  chalcedony4.  Fresh  andesite- 
glasses  also  occur  at  Santorin5,  reproducing  the  perlitic 
fissures  and  other  features  of  the  acid  obsidians.  These 
seem  to  be  in  the  main  hornblende-bearing,  but  contain  augite 
associated  with  that  mineral.  Vesicular  and  pumiceous 
modifications  are  found. 

Among  British  Tertiary  rocks  the  'pitchstone'  of  the 
Sgurr  of  Eigg6  and  the  neighbouring  islet  Hysgeir7  has 
apparently  the  composition  of  a  dacite.  The  prominent 
phenocrysts  are  of  sanidine,  or  perhaps  anorthoclase,  and  are 
sometimes  much  corroded  by  the  ground-mass.  A  little 
green  augite  and  magnetite  also  occur.  The  ground  is  a 
brown  glass  rich  in  crystallitic  growths  or  in  minute  felspar- 
microlites.  Of  older  rocks,  that  described  by  Prof.  Judd8 
from  the  (probably)  Old  Red  Sandstone  breccia  near  Scroggie- 

1  G.  M.  1894,  551-553  (Carrock  Fell  dykes) ;  Heed,  Q.  J.  G.  S.  (1895) 
li,  183-187,  pi.  vi,  figs.  6,  7  [2292,  2293]  (Fishguard). 

2  The  name  was  first  used  by  Stache  for  quartz-bearing  andesites  in 
Transylvania  (Dacia). 

3  Fouque  and  Levy,  pi.  xvm.  4  Ibid.  pi.  xvn,  fig.  2. 

5  Ibid.  pi.  xxx ;  xxxi,  fig.  1. 

6  Judd,  Q.  J.  G.  S.  (1890)  xlvi,  380. 

7  Q.  J.  G.  S.  (1896)  Hi,  372. 

8  Q.  J.  G.  S.  (1886)  xlii,  427-429,  pi.  xin,  figs.  7,  8. 


186  DACITES,   ETC. 

side  Farm  in  N.E.  Fife  is  perhaps  rather  on  the  border-line 
between  rhyolite  and  dacite.  It  has  a  glassy  modification, 
which  the  author  styles  mica-dacite-glass.  Phenocrysts  of 
oligoclase  and  deep  brown  biotite  are  embedded  in  a  glassy 
ground-mass  containing  trichites,  globulites,  and  imperfect 
microlites  of  felspar  (perhaps  orthoclase).  The  glass  shews 
beautiful  perlitic  fissures.  Little  is  known  of  true  dacites 
among  the  Lower  Palseozoic  lavas  of  this  country,  though 
some  of  the  rocks  included  above  as  rhyolites  would  probably 
be  styled  dacites  by  certain  petrologists.  The  name  has  also, 
as  remarked  above,  been  applied  loosely  to  some  of  the  acid 
intrusives. 

A  number  of  dacites  were  described  from  Nevada  by  Zirkel1, 
and  some  of  Richthofen's  'glassy  rhyolites'  from  the  same 
region  seem  to  belong  rather  to  this  family'2.  Dacites  are 
also  well  represented  among  the  Tertiary  and  Recent  lavas 
in  California,  Oregon,  and  Washington,  and  in  San  Salvador3. 
Biotite  is  prominent  among  the  ferro-magnesian  minerals,  and 
sometimes  hornblende.  At  Lassen's  Peak  in  California4  occurs 
a  type  rich  in  phenocrysts,  which  consist  of  plagioclase  felspar, 
biotite,  hornblende,  and  quartz,  while  the  ground-mass  is 
essentially  of  glass  (fig.  41,  A).  This  is  one  of  the  original 
'  nevadites,'  and  most  of  the  rocks  so  styled  are  probably  to  be 
classed  as  dacites. 

The  andesites  characterized  by  biotite  or  hornblende  have 
affinities,  as  already  remarked,  with  the  typical  trachytes.  A 
mica-andesite  free  from  hornblende  is  exceptional,  but  the 
name  may  be  applied  to  varieties  in  which  biotite  is  the 
dominant,  though  not  the  sole,  ferro-magnesian  constituent. 
The  rocks  usually  taken  as  the  type  of  hornblende-andesite5 
are  those  of  the  Tertiary  volcanic  district  of  the  Siebengebirge, 
near  Bonn,  already  alluded  to  as  the  home  of  certain  typical 

1  Micro.  Petrogr.  Fortieth  Parallel  (1876),  134-142  :  see  also  Iddings, 
Monog.  xx    U.  S.  Geol.  Sur.  (1893)   308-373   (Eureka  district),  and  in 
Diller,  215-217. 

2  Hague  and  Iddings,  A.  J.  S.  (1884)  xxvii,  460,  461. 

3  Ibid.  (1886)  xxxii,  29,  30. 

4  Ibid.  (1883)  xxvi,  231-233 ;  Diller,  217-219. 

5  For  coloured  figures  of  several  French  examples  see  Fouqu6  and 
Levy,  pi.  xxn,  xxvm,  xxix,  xxxvm. 


HORNBLENDE-ANDESITES. 


187 


trachytes.     In  addition  to  abundant  brown  hornblende,  these 
andesites  contain  more  or  less  biotite  and  a  few  prisms  or 


B 


FIG.  41.     ANDESITIC  LAVAS,  CALIFORNIA  :    x  22. 

A.  Dacite,  Lassen's  Peak.     The  phenocrysts  are  of  andesine  (some 
with   large   glass-inclusions),    hornblende,    biotite,    and   magnetite.     In 
parts  of  the  slide,  not  figured,  quartz,  sanidine,  and  pyroxene  occur 
more  sparingly.     The  ground-mass  is  a  clear  glass  crowded  with  little 
acicular  crystallites.     There  are  also  growths  analogous  to  spherulites, 
but  with  only  very  imperfectly  radiate  structure  [2800], 

B.  Hornblende- Andesite,  Mt  Shasta.     The  phenocrysts  are  of  horn- 
blende (with  resorption-border)  and  zoned  labradorite.    The  ground-mass 
consists  of  little  microlites,  chiefly  of  andesine  [2802]. 

grains  of  pale  green  augite.  The  two  former  minerals  always 
shew  the  phenomenon  of  resorption.  The  Bolvershahn  rock, 
with  a  considerable  amount  of  deep  brown  biotite,  may  be 
called  a  hornblende-mica-andesite.  The  felspar  phenocrysts 
shew  very  marked  zonary  banding  in  polarized  light.  The 
Wolkenburg  rock  is  a  characteristic  hornblende-andesite.  Its 
phenocrysts  include  the  three  ferro-magnesian  minerals  men- 
tioned, hornblende  largely  predominating,  good  crystals  of 
andesine,  and  a  little  magnetite  and  apatite,  while  its  ground- 


188  HORNBLENDE- ANDESITES. 

mass  is  of  the  trachytic  type.  A  very  similar  rock  is  that  of 
Stenzelberg,  in  which  some  of  the  hornblende  crystals  attain 
a  conspicuous  size. 

In  America  Iddings1  has  recorded  mica-andesites,  horn- 
blende-mica-andesites,  and  hornblende-pyroxene-andesites  from 
the  Tewan  Mts  in  New  Mexico.  These  rocks  have  a  glassy 
base.  Similar  examples  come  from  Lassen's  Peak  (Gal.), 
Mt  Hood  (Ore.),  and  Mt  Rainier  (Wash.)2.  The  phenocrysts 
often  shew  parallel  intergrowths  of  hornblende,  augite,  and 
hypersthene.  The  'trachytes'  of  ZirkeP  and  others  in  the  Great 
Basin  and  elsewhere  are  in  part  hornblende-mica-andesites4. 
This  type  occurs  with  others  at  the  Comstock  Lode5,  and  an 
example  with  beautifully  zoned  felspar  phenocrysts  has  been 
described  by  Iddings6  from  the  Eureka  district.  Others  occur 
in  the  Sierra  Nevada  of  California7.  In  these  districts  horn- 
blende-andesites  free  from  mica  are  also  found,  and  a  good 
example  of  this  type  comes  from  Mount  Shasta,  Cal. 
(fig-  41,  B). 

In  our  own  country  these  rocks  are  very  poorly  represented. 
One  good  example  occurs  on  the  summit  of  Beinn  Nevis8,  and, 
though  probably  of  Carboniferous  age,  it  is  fairly  fresh.  The 
phenocrysts  are  of  light-brown  idiomorphic  hornblende  and  a 
plagioclase  full  of  glass-inclusions,  etc.  The  ground-mass  is 
obscured  by  specks  of  iron-ore  and  alteration-products,  but 
is  seen  to  consist  largely  of  densely  packed,  minute  felspar- 
microlites.  Little  is  known  of  hornblende-andesites  among 
the  Lower  Palaeozoic  and  pre-Palseozoic  lavas  of  Britain  or  of 
America.  An  Ordovician  hornblende-andesite  of  somewhat 
basic  composition  occurs  near  Kildare9,  and  others,  brecciated 

1  Bull.  No.  66  U.  S.  Geol.  Sur.  (1890)  12-16. 

2  Iddmgs,  12th  Ann.  Rep.  U.  S.  Geol.  Sur.  (1892)  610-612,  pi.  LI  :  see 
also  Diller,  221-223  (Mt  Shasta,  Cal.). 

3  Micro.  Petrogr.  Fortieth  Parallel  (1876),  143-162. 

4  Hague  and  Iddings,  A.  J.  S.  (1883)  xxvi,  460. 

5  Hague  and  Iddings,  Bull.  No.  17  U.  S.  Geol.  Sur.  (1885)  23. 

6  Monog.  xx  U.  S.  Geol.  Sur.  (1893)  364-368,  pi.  v,  figs.  1,  3,  4 ;  vi, 
tig.  2;  and  in  Diller,  219-221. 

7  Turner,  14th  Ann.  Rep.  U.  S.  Geol.  Sur.  (1894)  487,  488. 

8  Teall,  pi.  xxxvn,  fig.  1. 

»  Beynolds  and  Gardiner,  Q.  J.  G.  S.  (1896)  lii,  602. 


HYPERSTHENE-ANDESITES.  189 

and  altered,  are  found  on  Slieve  Gallion,  co.  Londonderry1. 
Good  examples  occur  in  Minnesota2. 

Andesites  having  a  pyroxene  as  their  dominant  non-fel- 
spathic  constituent  are  perhaps  more  widely  distributed  than 
any  other  group  of  lavas,  and  are  largely  represented  among 
the  products  of  volcanoes  now  active.  Since  a  rhombic  and  a 
monoclinic  pyroxene  are  often  associated,  the  rocks  are  spoken 
of  as  pyroxene-andesites,  while  the  marked  predominance  of 
one  or  other  of  these  minerals  gives  a  hypersthene-  or  an 
augite-andesite.  Hypersthene-andesites,  and  hypersthene- 
augite-andesites  in  which  the  rhombic  pyroxene  predominates 
over  the  monoclinic,  are  especially  widely  distributed  among 
the  lavas  of  different  periods.  Prof.  Judd3  has  pointed  out 
that  the  same  general  petrographical  type  is  found  in  lavas 
ranging  in  chemical  composition  from  basalt  to  dacite.  Thus 
the  basic  dykes  of  Santorin,  the  lava  of  Buffalo  Peaks  in 
Colorado,  the  Cheviot  rocks,  the  recent  lavas  of  Santorin,  and 
the  rocks  of  Krakatau  consist  of  the  same  minerals  in  a  glassy 
base  of  the  same  general  composition,  but  the  relative  pro- 
portions of  the  minerals  (in  the  aggregate  basic)  to  glass 
(decidedly  acid)  varies  in  the  different  cases  from  9  :  1  to 
1  :  9.  This  illustrates  the  impossibility  of  naturally  classifying 
by  mineralogical  characters  alone  rocks  which  have  a  glassy 


The  wide  distribution  of  hypersthene-andesites  in  Europe 
and  America  was  first  insisted  upon  by  Whitman  Cross4,  who 
shewed  that  in  a  very  large  number  of  andesitic  lavas  hyper- 
sthene had  previously  been  mistaken  for  augite.  The  rock 
upon  which  his  first  observations  were  made  was  from  Buffalo 
Peaks,  Colorado.  The  '  augite-andesites '  of  Zirkel5  from 
Nevada  have  both  rhombic  and  monoclinic  pyroxenes,  but 


1  Cole,  Sci.  Tr.  Roy.  Dull.  Soc.  (1897)  vi,  222,  etc. 
"  Wads-worth,  Bull.  No.  2  Geol.  Sur.  Minn.  (1887)  pi.  x,  xi;  Grant, 
2lst  Ann.  Eep.  Geol.  Sur.  Minn.  (1894)  57,  58. 

3  G.  M.  1888,  1-11.     For  illustrations  of  the  Krakatau  rocks  see  Eep. 
Krak.  Comm.  Eoy.  Soc.  (1888)  pi.  in. 

4  Bull.  No.  1  U.  S.  Geol.  Sur.  (1883);  A.  J.  S.  (1883)  xxv,  139;  and 
in  Diller,  224-227. 

5  Micro.  Petrogr.  Fortieth  Parallel  (1876),  221-227,  pi.  xi,  fig.  2. 


190  BRITISH    PYROXENE-ANDESITES. 

the  former  predominates1,  and  true  augite-andesites  seem  to 
be  unrepresented  among  the  lavas  of  the  Great  Basin  region. 
Hypersthene-andesites  occur  in  great  variety2  among  the 
Recent  lavas  of  Mt  Shasta  (Cal.)3,  Mt  Rainier  (Wash.)4,  etc. 
These  are  crowded  with  phenocrysts  of  zoned  plagioclase  and 
pyroxenes,  hypersthene  predominating  over  augite,  while  the 
ground-mass  varies  from  holpcrystalline  to  vitreous.  Andesites 
carrying  hornblende  in  addition  to  hypersthene  occur  in  the 
Eureka  district5,  the  Sierra  Nevada6,  etc. 

Pyroxene-andesites  are  abundant  among  the  older  volcanic 
rocks  of  Britain.  Some  in  the  Lake  District  contain  plenty 
of  pseudomorphs  after  a  rhombic  pyroxene  (Falcon  Crag  near 
Keswick,  etc.),  while  many  others  are  characterized  by  mono- 
clinic  pyroxene  only.  A  few  of  these  rocks  have  been  described 
by  Mr  Clifton  Ward,  Prof.  Bonney,  and  Mr  Hutchings7.  The 
ground-mass  is  usually  typically  hyalopilitic. 

In  the  Bala  series  of  Caernarvonshire  there  are  few  an- 
desites.  Some,  with  augite  only,  occur  in  the  Lleyn  district8, 
and  one  with  dominant  hypersthene  forms  an  intrusive  mass 
at  Cam  Boduan9  in  the  same  district.  The  andesites  of  the 
Stapeley  Hills  (Todleth,  etc.)  in  Shropshire  are  of  the  same 
general  type  as  the  Cheviot  rocks,  containing  both  rhombic 
and  monoclinic  pyroxenes,  and  this  is  true  also  of  the  Bala 
lavas  of  the  Breidden  Hills  (Moel-y-golfa,  etc.)10.  Pyroxene- 
andesites  of  Bala  age  are  known  at  various  localities  in 
Ireland  ;  e.g.  Lambay  Is.11  and  Portraine12. 


1  Cross,  I.e. ;  Hague  and  Iddings,  A.  J.  S.  (1884)  xxvii,  457-460. 

2  Hague  and  Iddings,  A.  J.  S.  (1883)  xxvi,  222-235. 

3  Ciller,  227,  228,  pi.  xxxn. 

4  G.  O.  Smith,  18th  Ann.  Rep.  U.  S.  G.  S.  part  IT  (1898),  418-420. 

5  Iddings,  Monog.  xx  U.  S.  Geol.  Sur.  (1893)  348-364,  pi.  vir,  fig.  1. 

6  Turner,  Uth  Ann.  Rep.  U.  S.  Geol.  Sur.  (1894)  488;  and  17th  Ann. 
Rep.  (1897)  617-619,  pi.  XLV,  fig.  A. 

7  G.  M.  1891,  539-544. 

8  Bala  Vole.  Ser.  Caern.  68.  9  Ibid.  69-71. 

10  Watts,  Q.  J.  G.  S.  (1885)  xli,  539-543  ;  Proc.  Geol.  Asxoc.  (1894) 
xiii,  337-339,  with  figures. 

11  Gardiner  and  Reynolds,  Q.  J.  G.  S.  (1898)  liv,  142-145. 

12  Ibid.  (1897)  liii,  521-527 ;  Sollas,  Pr.  Geol.  Ass.  (1893)  xiii,  100, 
with  fig.  6, 


CHEVIOT   PYROXENE-ANDESITES.  191 

Many  of  the  old  lavas  loosely  grouped  under  the  field- 
term  'porphyrite'  in  the  Old  Red  Sandstone  and  Carboni- 
ferous of  Scotland  are  andesites,  ranging  in  composition  from 
a  relatively  acid  type  (dacite)  to  varieties  verging  on  basalt. 
One  of  the  former,  from  North-east  Fife,  has  already  been 
mentioned.  In  the  same  district  are  good  examples  of  more  basic 
types  also  (Northfield  and  Causeway  Head)1.  From  Dumyat  and 
elsewhere  in  the  Ochils2  come  typical  pyroxene-andesites  with 
both  hypersthene  and  augite,  the  former  predominating.  The 
freshest  type  has  an  unaltered  glassy  base,  which  in  other 
varieties  is  devitrified.  The  Old  Red  Sandstone  lavas  of  the 
Cheviots3  are  mostly  hypersthene-andesites,  containing  both 
rhombic  and  monoclinic  pyroxenes.  The  freshest  type  shews 
phenocrysts  of  labradorite,  often  honeycombed  with  inclusions 
of  ground-mass,  crystals  of  hypersthene  shewing^  distinct 
pleochroism,  and  crystals  and  grains  of  pale  augite,  in  a 
ground-mass  of  pale  brown  glass  and  felspar  microlites 
(fig.  42).  The  ground  sometimes  has  flow-structure,  and 
shews  varieties  of  the  hyalopilitic  type.  The  iron-ores  are 
represented  by  magnetite  and  minute  red  scales  of  haematite. 
The  rock  is  often  veined  by  opal  or  chalcedony,  stained  red 
with  ferric  oxide.  The  more  weathered  lavas  of  the  district 
(part  of  the  '  porphyrites '  of  some  authors)  have  had  similar 
characters,  but  the  felspars  and  pyroxenes  are  more  or  less 
decomposed,  and  the  ground  obscured  by  ferruginous  matter. 
There  are  sometimes  vesicles,  filled  with  chalcedony,  etc. 
Fresh  examples  come  from  Kilham,  Longknowe,  Haddan,  and 
Coldsmouth  Hills. 

Certain  dykes  described  by  Mr  Teall4  in  the  North  of 
England  may  be  referred  to  here,  being  petrographically 
augite-andesites.  Some  of  them  (Cleveland,  etc.}  must  be  of 
Tertiary  age,  while  others  are  possibly  late  Paleozoic.  The 

1  Judd,  Q.  J.  G.  S.  (1886)  xlii,  425-427,  pi.  xin,  figs.  1,  2. 

2  Flett,  Tr.  Edin.  O.  S.  (1897)  vii,  290-297,  pi.  xvn ;  Watts  in  Geikie's 
Ancient  Volcanoes  (1897),  i,  274-276. 

a  Teall,  pi.  xxxvi,  xxxvn,  fig.  2;  G.  M.  1883,  102-106,  146-152, 
pi.  iv,  252-254;  Petersen,  G.  M.  1884,  226-234  (Abstr.);  Watts,  Mem. 
Geol.  Stir.  Eng.  and  Wales,  Expl.  of  Quarter-sheet  110  S.  W.,  N.  S.  sheet  3 
(1895),  12,  13. 

*  Q.  J.  G.  S.  (1884)  xl,  209-247,  pi.  xn,  xm ;  Brit.  Petr.  pi.  xn,  xiv. 


192  BRITISH    AUGITE-ANDESITES. 

Cleveland  dyke  is  traced  from  near  Whitby  to  Armathwaite 
near  Carlisle,  and  perhaps  farther.  It  contains  porphyritic 
felspars,  often  broken,  in  a  ground-mass  composed  of  small 


FIG.  42.     HYPERSTHENE-ANDESITE,  CHEVIOT  HILLS, 
NOBTHUMBERLAND  ;    X  20. 

Phenocrysts  of  labradorite  and  hypersthene  enclosed  in  a  fine-textured 
ground- mass  with  a  large  proportion  of  glassy  base  [2762]. 

felspar  crystals,  minute  crystals  and  grains  of  augite,  crystals 
of  magnetite,  and  abundant  interstitial  matter.  This  last 
is  sometimes  glassy,  but  commonly  charged  with  various 
products  of  devitrification,  giving  a  decided  reaction  with 
polarized  light.  The  Acklington  dyke  is  similar,  but  usually 
without  the  porphyritic  crystals  and  with  less  of  the  inter- 
stitial base.  The  Tynemouth  dyke  is  less  fine-textured.  It 
contains  porphyritic  aggregates  of  anorthite  crystals  in  a 
ground-mass  of  elongated  lath-shaped  felspars,  grains  of  augite, 
magnetite,  and  a  considerable  amount  of  interstitial  base  with 
devitrification-products  and  microlites  and  skeletons  of  fel- 
spar1. The  dykes  of  Hebburn,  Brunton  (fig.  43),  Seaton,  and 

1  The  structure  is  the  '  intersertal '  of  Rosenbusch,  who  cites  these 
dykes  as  examples  of  his  '  tholeiite.' 


BRITISH   AUGITE-ANDESITES.  193 

Hartley  are  similar,  though  usually  without  the  large  felspars. 
Prof.  Judd1,  who  has  described  dykes  of  this  type  in  Arran, 


FIG.  43.     AUGITE-ANDESITE,  BRUNTON  DYKE,  BINGFIELD, 
NORTHUMBERLAND  ;    x  20. 

The  only  minerals  seen  are  felspar  and  augite.  There  are,  in  addition, 
interstitial  patches  of  brown  glass,  which  enclose  abundant  crystallitic 
growths.  The  structure  is  typically  intersertal  [2359]. 

remarks  as  characteristic  of  them  the  tendency  of  the  glassy 
residue  to  become  separated  from  the  crystalline  portion  of 
the  rock,  either  as  a  selvage,  or  as  a  central  band,  or  in 
irregular  patches  and  strings  (Eskdalemuir  in  Dumfriesshire)2, 
or,  again,  wholly  or  partially  filling  vesicles  in  the  rock,  as 
already  remarked  by  Mr  Teall3  in  the  Tynemouth  dyke. 
Different  dykes  belonging  to  this  group  vary  much  as  regards 
the  relative  proportions  of  the  crystalline  elements  and  the 
glassy  residue.  Specimens  of  various  types  may  be  found  in 
Arran,  the  Cumbrae  Isles,  Skye,  Donegal4,  etc. 

1  Q.  J.  G.  S.  (1893)  xlix,  541. 

2  Geikie,  Proc.  Eoy.  Phys.  Soc.  Edin.  (1880)  v,  244-252,  pi.  v,  vi ; 
Teall,  p.  196,  pi.  xxiv,  fig.  1. 

3  G.  M.  (1889)  481-483,  pi.  xiv. 

4  Sollas,  Sci.  Pr.  Eoy.  Dubl.  Soc.  (1893)  viii,  91-93. 

H.    P.  13 


CHAPTER  XIV. 

BASALTS. 

IN  the  basalt  family  we  include  all  the  basic  lavas  except 
those  in  which  a  relatively  high  content  of  alkalies  has  given 
rise  to  the  formation  of  minerals  of  the  felspathoid  group. 
The  rocks  range  in  texture  from  vitreous  to  holocrystalline. 
Except  in  a  few  of  the  latter  (dolerites),  the  distinction  be- 
tween phenocrysts  and  ground-mass  is  commonly  well  marked, 
but  the  relative  proportions  of  the  two  vary  greatly  in  dif- 
ferent types.  The  characteristic  minerals  in  this  family  of 
rocks  are  a  felspar  rich  in  lime,  augite,  and  olivine. 

Following  our  principle,  we  shall  make  no  distinction,  as 
regards  nomenclature  and  classification,  between  Tertiary  and 
pre-Tertiary  lavas.  Foreign  petrologists  usually  restrict  the 
names  basalt  and  dolerite  to  the  newer  examples,  their  older 
equivalents  being  denoted  by  such  names  as  melaphyre,  augite- 
porphyrite,  diabase,  etc.,  some  of  which  are  also  applied  to 
rocks  of  the  hypabyssal  division. 

Certain  exceptional  lavas  (limburgites,  etc.}  which  are  of 
ultrabasic,  rather  than  normally  basic,  composition  will  be 
briefly  noticed.  Some  of  them  probably  correspond  rather  with 
the  nepheline-basalts,  etc.,  treated  in  the  succeeding  chapter. 

Constituent  minerals.  The  felspars  of  the  basalts  are 
of  decidedly  basic  varieties.  When  distinctly  porphyritic 
crystals  occur,  they  seem  to  be  usually  bytownite  or  anorthite, 
while  the  felspars  of  the  ground-mass  are  more  commonly 
labradorite.  The  phenocrysts  shew  albite-lamellation,  often 


MINERALS  OF  BASALTS.  195 

combined  with  pericline-  and  carlsbad-twinning.  Zonary 
structure  and  zonary  arrangement  of  glass-cavities  are  met 
with.  The  felspars  of  the  ground-mass  have  the  lath-shape, 
and  are  commonly  too  narrow  to  shew  repeated  twinning. 
Orthoclase  is  found  only  in  certain  abnormal  types. 

The  dominant  pyroxenic  constituent  is  an  ordinary  augite, 
and  this  too  may  occur  in  two  generations.  If  so,  the  pheno- 
crysts  often  have  jgood  crystal-forms,  with  octagonal  cross- 
section  :  twinning  is  frequently  seen1,  and  sometimes  zoning 
and  hour-glass  structure.  The  colour  is  usually  very  pale, 
brownish  or  more  rarely  greenish,  the  latter  especially  in  the 
interior  of  a  crystal.  The  augite  of  the  ground-mass  is  either 
in  little  idiomorphic  prisms  or  in  granules,  and  is  often  very 
abundant.  Decomposition  of  the  augite  produces  chloritic 
substances,  etc.2  A  rhombic  pyroxene,  hypersthene  or  bronz- 
ite,  occurs  only  in  certain  basalts,  where  it  seems  to  some 
extent  to  take  the  place  of  olivine.  It  is  always  in  idio- 
morphic prisms,  and  in  the  older  rocks  is  very  generally 
serpentinized.  Some  basalts,  again,  contain  corroded  crystals 
of  brown  hornblende,  and  others  a  little  brown  mica. 

In  the  greater  part  of  the  basalts  olivine*  is  an  essential 
constituent,  and  in  many  it  is  abundant,  though  confined, 
as  a  rule,  to  phenocrysts.  These  are  sometimes  well  shaped 
crystals,  sometimes  more  or  less  rounded,  while  in  some  of 
the  more  glassy  rocks  hollow  or  skeleton  crystals  and  crystal- 
lites occur4.  The  mineral  is  colourless  or  very  pale  green. 
It  often  shews  serpentine-strings  following  cleavage-  or  other 
cracks5,  and  with  further  alteration  passes  into  various 
secondary  products,  serpentine,  carbonates,  etc.  Another 
common  change  is  the  production  of  a  red  or  brown  margin 
to  the  olivine,  due  to  iron-oxide,  the  olivine  in  basalts,  and 
still  more  in  limburgites,  being  often  of  a  variety  rich  in 
iron6.  Another  mode  of  alteration  sometimes  met  with  results 

Cohen  (3),  pi.  xxiv,  fig.  3 ;  xxv,  fig.  1. 

Teall,  pi.  xxn,  fig.  2. 

Cohen  (3),  pi.  XLVIII,  fig.  2. 

Ibid.  pi.  i,  fig.  3  ;  xiv,  fig.  2. 

Zirkel,  Micro.  Petr.  Fortieth  Parall.,  pi.  x,  fig.  3 ;  xi,  fig.  3. 

Cohen  (3),  pi.  LXI,  fig.  1. 

13—2 


196  MINERALS   OF   BASALTS. 

in  the  formation  of  brown  pleochroic  pseudomorphs  of  a 
mineral  with  a  perfect  cleavage  and  the  appearance  of  a 
mica.  It  seems  to  agree  in  general  characters  with  the 
mineral  described  in  California  by  Lawson1  under  the  name 
iddingsite ;  but  the  author  named,  regarding  this  as  an 
original  constituent,  has  made  it  the  characteristic  of  a  new 
group  of  lavas  (carmeloites). 

Octahedra  and  grains  of  magnetite  are  generally  abundant, 
and  this  mineral  frequently  recurs  in  a  second  generation 
in  little  granules.  Besides  this,  there  are  frequently  little 
opaque  or  deep  brown  scales  of  ilmenite  or  deep  red  flakes 
of  hcematite.  Grains  of  native  iron  occur  locally  in  a  few 
basalts  (Ovifak  in  Disco,  Greenland)2. 

Of  other  common  minerals  we  need  note  only  apatite, 
forming  long  needles,  either  colourless  or  of  a  faint  violet 
or  bluish  tint. 

A  peculiar  feature  in  certain  American  basalts3  is  the 
occurrence  of  isolated  grains  of  quartz.  These  are  always 
corroded  by  the  magma  and  generally  surrounded  by  a  ring 
of  augite  or  its  alteration-products,  a  character  usually 
associated  with  foreign  quartz-grains  picked  up  by  a  basic 
magma.  In  this  case,  however,  there  is  reason  to  believe 
that  the  mineral  is  an  original  constituent  formed  under 
peculiar  conditions.  It  is  comparable  with  similar  grains 
found  in  many  lamprophyres  (see  above,  p.  145). 

Structures.  The  rocks  of  the  basalt  family  present  a 
wide  range  of  characters,  from  purely  glassy  examples  at 
one  extreme  to  wholly  crystalline  at  the  other.  Rocks  ex- 
hibiting such  a  range  may  occur,  perhaps  exceptionally,  in 
one  district,  their  petrological  characters  being  correlated 
with  their  various  modes  of  occurrence,  as  is  well  described 
by  Prof.  Judd4.  On  the  whole,  the  tendency  to  crystallization 

1  Bull.  Geol.  Dep.  Univ.  Gal.  (1893)  i,  29-46,  pi.  iv. 

2  Fouque  and  Levy,  pi.  xxxvi,  fig.  2 ;  Steenstrup,  M.  M.  i,  148,  pi.  vi. 

3  Ciller,   252,  253,  pi.   xxvn;    A.  J.  S.   (1887)   xxxiii,   45-49;    Bull. 
No.  79  U.  S.  Geol.  Sur.  (1891)  24-29;  ladings,  A.  J.  S.  (1888)  xxxvi, 
209-213 ;  Bull.  No.  66  U.  S.  Geol.  Sur.  (1890)  16-31 ;  Monog.  xx.  U.  S. 
Geol.  Sur.  (1893)  393,  pi.  iv,  fig.  1. 

4  Q.  J.  G.  S.  (1886)  xlii,  66-82,  pi.  v,  vi. 


GLASSY  STRUCTURE  IN  BASALTS.         197 

is  much  stronger  here  than  in  the  more  acid  families  of  lavas. 
Again,  the  order  of  crystallization  of  the  several  constituents 
is  less  strongly  marked,  the  mutual  relations  between  augite 
and  felspar,  in  respect  of  priority,  varying,  while  the  iron- 
ores,  though  they  commonly  begin  to  crystallize  at  an  early 
stage,  may  be  in  part  rather  late.  These  remarks  are  true 
of  both  the  '  intratelluric '  and  the  '  effusive '  periods,  when 
these  are  distinctly  separable ;  but  in  some  of  the  holocrystalline 
types  the  porphyritic  character  is  not  recognizable.  Some  of 
these  rocks  differ  in  no  essential  from  those  already  described 
as  diabases,  the  petrological  distinction  between  the  hypabyssal 
and  the  volcanic  types  not  being  marked  by  any  hard  and 
fast  line. 

Except  in  the  form  of  lapilli  and  fragments  in  tuffs,  the 
purely  vitreous  type,  tachylyte,  is  of  very  limited  distribution, 
being  found  only  as  a  very  thin  crust  on  some  lava-flows  or 
a  narrow  selvage  to  basalt-dykes.  It  consists  of  a  brown 
or  yellow  glass  densely  charged  with  a  separation  of  magnet- 
ite1. This  is  sometimes  in  globulites2  disseminated  through 
the  glass  so  as  to  render  it  almost  opaque,  or  collected  in 
cloudy  patches  (cumulites) ;  at  other  times  it  forms  trichites 
or  crystallites  of  minute  size3.  Perlitic  structure  is  less 
common  than  in  the  obsidians.  Interesting  spherulitic  struct- 
ures are  met  with  in  some  examples4.  When  distinct  pheno- 
crysts  occur  abundantly  in  the  glassy  ground-mass,  we  have 
what  is  sometimes  called  the  '  vitrophyric '  structure5.  The 
basic  glass  is  subject  to  secondary  changes,  probably  involving, 
as  a  rule,  hydration  and  other  chemical  changes ;  but  the 
resulting  substance,  known  as  palagoriite,  is  still  an  isotropic 
glass,  yellow,  brown,  or  sometimes  green  in  sections. 

Radiate  aggregates  of  felspar  microlites  or  fibres,  answering 
to  the  spherulites  of  acid  rocks,  occur  in  some  basaltic  glasses, 
which  are  known  as  variolites.  These  aggregates  vary  in  size 
and  in  the  regularity  of  their  structure,  which  ranges  from 

1  Cohen  (3),  pi.  xxxix,  figs.  1,  2. 

2  Ibid.  pi.  vi,  fig.  4 ;  iv,  fig.  2. 

3  Judd  and  Cole,  Q.  J.  G.  S.  (1883)  xxxix,  pi.  xiv. 

4  Cole,  ibid.  (1888)  xliv,  300-307,  pi.  xi. 

5  See  chromolith.,  Berwertb,  Lief,  n  (Rhon). 


198        HYPOCRYSTALLINE   STRUCTURE   IN   BASALTS. 

mere  fan-like  and  sheaf-like  groupings  (cf.  fig.  44,  A)  to 
spherules  with  a  perfect  radiate  structure.  They  may  occur 
isolated  in  a  glassy  matrix,  or  coalesce  into  bands,  or  form 
a  densely  packed  mass  with  little  or  no  interstitial  matter. 
The  variolites  are  very  susceptible  to  alteration. 


B 


cuw 


FIG.  44. 

A.  Andesite  vein  approaching  the  structure  of  variolite,  Carrock 
Fell,  Cumberland;  x  20,  crossed  nicols.  This  is  of  the  type  which 
consists  essentially  of  radiating  felspar  fibres  grouped  in  sheaf-like 
bundles.  There  are  also  skeleton-prisms  of  a  pyroxenic  mineral,  better 
seen  in  natural  light  [1552].  B.  Limburgite,  Whitelaw  Hill,  Had- 
dington  ;  x  20,  natural  light.  Phenocrysts  of  olivine  (&/),  zoned  augite 
(cut),  and  magnetite  are  enclosed  in  a  ground-mass  of  glass  containing 
abundant  prisms  and  granules  of  augite  but  no  felspar.  The  glass, 
which  constitutes  the  bulk  of  the  ground,  varies  from  brown  to  nearly 
colourless  [1982]. 

Leaving  the  glassy  basalts,  we  note  those  in  which  the 
ground-mass  enclosing  the  phenocrysts  of  olivine,  augite, 
felspar,  etc.,  is  hypocrysialline,  consisting  of  lath-shaped  felspar- 
microlites  and  granules  or  microlites  of  augite  with  more 
or  less  of  a  residual  glassy  base.  Of  this  division  there  are 
various  types,  depending  on  the  relative  proportions  of  augite, 
felspar,  and  glass,  and  the  mutual  relations  of  the  minerals. 
When  the  felspar-microlites  preponderate,  usually  with  a 


HOLOCRYSTALLINE   STRUCTURE   IN   BASALTS.         199 

more  or  less  fluxional  arrangement,  the  ground-mass  does  not 
differ  essentially  from  the  *  hyalopilitic '  type  so  common  in 
the  pyroxene-andesites.  Vesicles  are  frequent  in  such  rocks. 
More  often,  however,  augite  is  abundantly  represented  in  the 
basaltic  ground-mass.  Again,  unindividualised  glass  may  form 
the  bulk  of  the  ground,  and  this  is  especially  the  case  in  the 
limburgites  (fig.  44,  B\  Another  type  of  structure,  already 
noticed  in  the  pyroxene-andesites,  is  the  intersertal,  in  which 
a  hypocry stall ine  or  glassy  ground-mass  occurs  only  as  an- 
gular patches  in  the  interstices  of  the  abundant  phenocrysts 
(compare  fig.  43). 

By  the  failure  of  the  glassy  residue  we  pass  to  those 
types  of  basalt  in  which  the  phenocrysts  are  enclosed  in  a 
holocrystalline  ground-mass.  Here  again  there  are  numerous 


aw ^^.  -^.^^^..^^     „.„.     , r,,,m^ 

e7 


FIG.  45.     BASALT,  ETNA  LAVA  OF  1669  ERUPTION,  CATANIA;    x20: 

shewing  phenocrysts  of  zoned  augite  (aw),  felspar  (/),  olivine  (ol), 
and  magnetite  (m)  in  a  holocrystalline  ground-mass  of  little  lath-shaped 
felspars  and  granules  of  augite  and  magnetite  [131]. 

varieties.  Sometimes  little  eye-like  or  lenticular  patches  re- 
latively rich  in  augite  are  contrasted  with  adjacent  patches 
rich  in  felspar.  When  felspar-microlites  make  up  a  large 


200  VARIOUS   STRUCTURES   IN    BASALTS. 

part  of  the  ground-mass,  we  have  a  structure  analogous  to 
the  *  pilotaxitic '  of  some  andesites  and  trachytes,  the  flow 
being  more  or  less  marked.  On  the  other  hand,  the  ground 
may  consist  mainly  of  small  rounded  granules  of  augite, 
between  which  the  little  felspars  seem  to  be  squeezed  (fig.  45). 

There  remain  the  types  distinguished  as  dolerites  (usually 
olivine-dolerites),  which,  in  the  most  typical  examples,  are 
holocrystalline  rocks  not  conspicuously  porphyritic,  sometimes 
of  coarse  texture  as  compared  with  the  generality  of  lavas. 
The  chief  structures  are  the  granulitic  and  the  ophitic,  the 
distinction  between  which  has  been  noticed  under  the  diabases 
(p.  134).  Typical  ophitic  structure  is  exceptional  in  most 
basaltic  regions.  Only  rarely  in  doleritic  lavas  do  we  find  an 
idiomorphic  development  of  the  augite. 

Some  dolerites  enclose  large  scattered  porphyritic  crystals 
of  felspar.  In  other  cases  there  are  porphyritic  aggregates 
of  crystals  (felspar,  olivine,  augite,  etc.}  having  the  mutual 
relations  characteristic  of  plutonic  rocks  :  this  is  the  glomero- 
porphyritic  structure  of  Prof.  Judd1.  It  is  not  confined  to 
the  holocrystalline  dolerites.  The  crystals  forming  such  a 
hypidiomorphic  aggregate  may  still  present  idiomorphic  out- 
lines towards  the  surrounding  rock2. 

Many  Tertiary  and  Recent  basalts  in  Germany,  Auvergne, 
and  other  regions  enclose  so-called  '  olivine-nodules^  which  are 
hypidiomorphic  aggregates  of  olivine  with  enstatite,  diopside, 
etc?  By  some  they  have  been  regarded  as  very  early  in- 
tratelluric  formations  from  the  magma,  by  others  as  actual 
enclosed  pieces  of  peridotites.  Such  nodules  are  not  found 
in  the  British  Tertiary  basalts. 

Leading-  types.  Some  basalts,  belonging  in  general  to 
the  less  basic  varieties,  are  free,  or  nearly  free,  from  olivine. 
These  rocks  usually  carry  a  rhombic  as  well  as  a  monoclinic 
pyroxene,  and  here,  as  in  some  other  families,  hypersthene 
may  be  considered  as,  to  some  extent,  taking  the  place  of 
the  more  basic  silicate  olivine.  Such  rocks,  which  may  be 

1  Q.  J.  G.  S.  (1886)  xlii,  71,  pi.  vn,  fig.  3. 

2  Teall,  ibid.  (1884)  xl,  235,  pi.  xm,  fig.  1. 

3  Fouqud  and  Levy,  pi.  XL,  fig.  1. 


HYPERSTHENE-BASALTS. 


201 


styled  hypersthene-basalts,  occur  among  the  Tertiary  lavas  of 
the  western  United  States.  Examples  have  been  noted  by 
Iddings1  from  the  Eureka  mining  district  in  Nevada.  The 
Ordovician  lavas  of  the  English  Lake  District  are  largely  of 
this,  type,  though,  as  already  noticed,  rhyolites  and  pyroxene- 
andesites  are  likewise  found.  Here  the  hypersthene  is  always 
converted  into  a  light  green,  pleochroic,  serpentinous  substance 
comparable  with  bastite.  The  most  striking  variety,  repre- 
sented at  Eycott  Hill2  and  numerous  other  localities  in  the 
district  and  at  Melmerby3  near  Cross  Fell,  has  large  rounded 


bo,-- 


FIG.  46.     HYPERSTHENE-BASALT,  EYCOTT  HILL  GROUP,  MELMERBY, 
CUMBERLAND  ;    x  20. 

To  the  right  is  one  of  the  large  crystals  of  labradorite  (Ib)  with  its 
peculiar  inclusions.  The  hypersthene  is  represented  by  bastite  pseudo- 
morphs  (ba) :  augite  occurs  in  less  abundance.  These,  with  the  little 
felspar-prisms,  the  granules  of  magnetite,  and  some  residual  glassy  base, 
make  up  the  bulk  of  the  rock  [1251]. 

phenocrysts  of  labradorite  with  carlsbad  and  albite-twinning. 
These  contain  rather  large  opaque  inclusions  in  the  form  of 

1  Monog.  xx  U.  S.  Geol.  Sur.  (1893)  386-394,  pi.  vn,  fig.  2. 

2  Ward,  Monthly  Micro.  Journ.  (1877)  xvii,  240-245 ;  Bonney,  G.  M. 
1885,  76-80;  Teall,  225-227. 

3  Q.  J.  G.  S.  (1891)  xlvii,  517. 


202  AMERICAN    OLIVINE-BASALTS. 

negative  crystals  and  smaller  enclosures  with  zonary  dispo- 
sition. In  other  varieties  of  the  lavas  these  large  crystals  are 
not  present.  The  ground-mass  consists  of  slender  striated 
prisms  of  plagioclase,  crystals  of  hypersthene  converted  to 
pleochroic  bastite,  granules  of  augite,  abundant  magnetite,  and 
an  isotropic  base  (fig.  46).  In  the  basic  lavas  of  the  Lake 
District  olivine  is  entirely  wanting.  Hypersthene,  pseudo- 
morphed  by  bastite,  is  frequently  present,  but  rarely  to  the 
exclusion  of  augite. 

Basalts  devoid  of  olivine  seem  to  be  widely  distributed 
among  the  Old  Red  Sandstone  lavas  of  Scotland1,  where  some 
of  them  have  been  styled  porphyrites. 

We  come  next  to  the  more  widely  distributed  olivine-basalts. 
Such  rocks  are  extensively  developed  among  the  lavas  of  late 
geological  age  in  America ;  for  instance,  in  the  Great  Basin 
region,  lying  between  the  Rocky  Mts  and  the  Sierra  Nevada. 
Here  they  are  mostly  porphyritic,  with  relatively  large  pheno- 
crysts  of  olivine,  plagioclase,  and  occasionally  augite,  in  a 
glassy,  microlitic,  or  microcrystalline  ground-mass.  A  smaller 
number  are  non-porphyritic,  consisting  of  a  uniform  aggregate 
of  plagioclase,  augite,  olivine,  and  magnetite,  often  with  a  con- 
siderable amount  of  glassy  base2.  Other  examples  have  been 
described  from  the  Sierra  Nevada3,  the  Tewan  Mts  (N.M.)4, 
and  San  Salvador5.  In  the  latter  region  it  has  been  remarked 
that  the  varieties  poor  in  olivine  carry  hypersthene  in  addition 
to  augite.  Recent  olivine-basalts  occur  at  many  localities  in 
Colorado,  New  Mexico,  Arizona,  and  about  Mt  Shasta  and 
Lassen's  Peak  in  California. 

The  Tertiary  basaltic  rocks  of  Britain,  as  developed  in  the 
Inner  Hebrides  and  in  various  parts  of  the  west  of  Scotland 
and  the  north  of  Ireland,  are  in  great  part  olivine-basalts. 

1  Peach   and  Home,   Tr.    Roy.  Soc.   Edin.   (1884)  xxxii,  379,  380; 
pi.  XLV,  figs.  1,  2  (Shetland). 

2  Hague  and  Iddings,  A.  J.  S.   (1884)  xxvii,  456,  457;   cf.  Zirkel, 
Micro.  Petr.  Fortieth  Parallel  (1876),  229-254 ;  pi.  x,  figs.  1,  3,  4 ;  xi, 
fig.  3. 

3  Turner,  Uth  Ann.  Rep.  U.  S.  Geol.  Sur.  (1894)  490-492. 
*  Iddings,  Bull.  No.  66  U.  S.  Geol.  Sur.  (1890)  16. 

5  Hague  and  Iddings,  A.  J.  S.  (1886)  xxxii,  27,  28. 


BRITISH   OLIVINE-BASALTS.  203 

They  have  been  well  described  by  Prof.  Judd1,  who  considers 
that  the  varied  series  of  structures  which  they  present 
constitute  intermediate  types  between  the  holocrystalline 
plutonic  rocks  at  the  one  extreme  and  the  glassy  basalts 
(tachylytes)  at  the  other.  He  distinguishes  two  parallel  lines 
of  transition.  One,  characteristic  of  the  true  extruded  lava- 
flows,  includes  the  '  granulitic '  dolerites  and  the  basalts  in 
which  the  augite  tends  to  form  granules  between  the  felspar 
prisms  ('microgranulitic'  structure).  The  other  series  of 
varieties  includes  the  ophitic  dolerites  and  the  micro-ophitic 
basalts,  in  which  the  augite  tends  to  enwrap  and  enclose  the 
felspars  :  this  seems  to  be  the  case  especially  in  intrusive 
members  of  the  group.  The  distinction  is  traceable  even  in 
those  basalts  which  consist  largely  of  a  glassy  base,  the  crystal- 
litic  growths  enclosed  in  the  glass  being  in  the  one  case  in  the 
form  of  granules  and  short  microlites,  often  rounded,  in  the 
other  case  in  the  form  of  skeleton-crystals  and  more  spreading 
growths.  In  the  true  lava-flows  both  granulitic  and  ophitic 
varieties  are  found,  but  the  former  are  the  more  common. 
Varieties  with  much  glassy  base  are  not  of  frequent  occurrence. 
An  amygdaloidal  structure  is  very  general,  and  the  most 
common  contents  of  the  amygdules  are  minerals  of  the  zeolite 
group. 

Some  of  the  Scottish  Tertiary  dolerites  and  basalts  are 
porphyritic,  the  felspar  occurring  in  two  generations,  of  which 
the  earlier  is  a  thoroughly  basic  variety,  sometimes  near 
anorthite,  while  the  latter  is  less  basic,  usually  labradorite. 
Porphyritic  augite,  however,  is  not  found,  and  this  feature 
distinguishes  the  group  of  rocks  in  question  from  the  Tertiary 
basalts  of  various  European  areas  and  also  from  many  Car- 
boniferous basalts  of  Scotland  and  Ireland.  The  corresponding 
rocks  of  the  Antrim  plateau  present  like  features.  As  described 
by  Prof.  Watts2,  some  of  the  basalts  have  porphyritic  felspars, 
but  most  are  of  quite  compact  character.  Olivine  grains  are 
enclosed  in  a  mass  of  elongated  felspar-crystals  and  granules 
of  augite,  with  occasionally  a  second  generation  of  smaller 
olivines. 

1  Q.  J.  G.  S.  (1886)  xlii,  49-95,  pi.  iv-vn ;  see  Teall,  pi.  x. 

2  Guide,  79. 


204  BRITISH   OLIVINE-BASALTS. 

The  basic  lavas  of  Carboniferous  age  in  this  country 
are  also  characteristically  olivine-bearing  rocks.  Those  of 
Derbyshire  (to  be  distinguished  from  the  ophitic  olivine- 
diabases  of  intrusive  habit)  are  porphyritic  olivine-basalts 
with  olivine  and  large  augite  phenocrysts  in  a  ground-mass 
of  small  felspar  laths,  augite  grains  and  prisms,  and  iron- ores, 
with  little  interstitial  matter  (Blackwell  Lane,  Great  Low)1. 

The  Kelso  lavas,  in  the  Lower  Carboniferous  of  the  Cheviot 
district,  are  olivine-basalts  with  phenocrysts  of  anorthite.  One 
from  Stichill  in  Roxburghshire  was  described  by  Mr  Teall*. 
In  other  examples,  from  Northumberland,  Prof.  Watts3  notes 
brown  pleochroic  pseudomorphs  after  olivine,  which  he  identifies 
with  iddingsite. 

The  Carboniferous  olivine-basalts  of  the  southern  half  of 
Scotland  present  a  considerable  variety  of  characters4.  The 
commonest  type  has  rather  abundant  small  olivines  and  grains 
of  augite  in  a  mesh  of  slender  felspars  with  microlitic  augite 
and  minute  granules  of  magnetite  (Dalmeny,  Bathgate  Hills, 
etc.}.  In  another  type  the  olivine  phenocrysts  are  large,  and 
the  felspar  microlites  are  found  only  in  small  amount  (lowest 
lavas  of  Bathgate  Hills,  Linlithgowshire).  A  well-known 
rock  from  the  Lion's  Haunch  on  Arthur's  Seat,  Edinburgh5, 
has  numerous  large,  well-built  crystals  of  augite,  olivine,  and 
felspar,  with  small  crystals  of  magnetite,  in  a  ground-mass  of 
little  crystals  and  microlites  of  felspar,  granules  of  augite  and 
magnetite,  and  some  residual  glass.  In  the  lava  of  Craig- 
lockhart  Hill  the  ground-mass  is  more  glassy,  while  the  pheno- 
crysts are  augite  and  olivine  without  felspar.  On  the  other 
hand,  there  is  a  holocrystalline  type,  which  is  an  olivine- 
dolerite  with  granulitic  to  sub-ophitic  structure  (Gallaston, 
N.W.  of  Kirkcaldy).  A  curious  variety,  very  rich  in  felspar, 

1  Arnold-Bemrose,  Q.  J.  G.  S.  (1894)  1,  624;   Pr.   Geol.  Ass.  (1899) 
xvi,  213,  214. 

2  G.  M.  (1883)  258-260,  pi.  vi. 

3  Mem.  Geol.  Sur.  Engl.  and  Wales,  Expl.  Quarter-sheet  110,  S.  W., 
N.  S.  sheet  3  (1895),   14. 

4  Geikie,  Q.  J.  G.  S.  (1892)  xlviii,  Proc.  105,  106;  Watts  in  Geikie's 
Ancient  Volcanoes  (1897),  i,  418,  and  Ann.  Hep.   Geol.  Sur.  for   1896, 
64,  65. 

5  Teall,  pi.  xxni,  fig.  1. 


BRITISH   OLIVINE-BASALTS.  205 

comes  from  Markle  quarry  in  the  Garlton  Hills,  Haddington- 
shire1.  Here  olivine  occurs  only  in  small  sporadic  grains, 
while  phenocrysts  of  labradorite  are  numerous,  and  the  ground- 
mass  consists  of  laths,  microlites,  and  granules  of  felspar  with 
dispersed  magnetite  and  probably  only  a  little  augite. 

A  rock  very  like  that  of  Lion's  Haunch  occurs  as  a  dyke 
near  the  Stack  of  Scarlet  in  the  south  of  the  Isle  of  Man2. 
The  phenocrysts  are  large  idiomorphic  crystals  of  fresh  plagio- 
clase  and  violet-brown  augite,  with  pseudomorphs  of  calcite 
and  serpentine  after  olivine.  The  ground-mass  is  of  lath- 
shaped  felspars,  augite,  and  iron-ores.  This  is  probably  con- 
nected with  the  Carboniferous  volcanic  series  of  the  Stack, 
which  consists  of  tuffs  with  dykes  and  probably  flows  of  a 
more  compact  basalt3.  The  latter  is  considerably  decomposed, 
the  augite  being  converted  into  chloritic  and  other  products. 
Porphyritic  felspars  occur,  and  the  little  lath-shaped  felspars 
of  the  ground-mass  shew  a  fluxional  arrangement.  The  much 
fresher  basalt,  which  forms  numerous  small  dykes  in  the  south 
of  the  Isle  of  Man4,  is  probably  of  Tertiary  age. 

In  the  neighbourhood  of  Limerick  is  a  considerable  de- 
velopment of  basaltic  lavas  of  Carboniferous  age.  These  differ 
from  the  Irish  Tertiary  basalts  in  various  points,  and  especially 
in  the  frequent  presence  of  augite  among  the  phenocrysts. 

Olivine-basalts  do  not  figure  largely  in  the  great  volcanic 
groups  which  characterize  the  Lower  Palaeozoic  in  various 
parts  of  Britain.  Sir  A.  Geikie5  has  noted  oli vine-basalts  of 
early  Cambrian  (or  late  pre-Cambrian)  age  near  St  David's 
(Rhosson,  Clegyr  Foig,  etc.).  The  idiomorphic  crystals  of 
olivine  in  these  rocks  are  replaced  largely  by  hematite.  The 
ground-mass  consists  of  augite-granules,  abundant  octahedra 
of  magnetite,  and  a  base  crowded  with  globulites  and  trichites, 
felspar  being  only  occasionally  recognized.  These  characters 
suggest  a  resemblance  to  the  limburgite  type,  noticed  below. 

1  Hatch,  Trans.  Roy.  Soc.  Edin.  (1892)  xxxvii,  119,  pi   i  fig  2 

2  Hobson,  Q.  J.  G.  S.  (1891)  xlvii,  443,  444. 

a  Ibid.  441.  4  Ibid.  445-447. 

5  Q.  J.  G.  S.  (1883)  xxxix,  304,  pi.  ix,  fig.  4.  On  basalts  from  Skomer 
Is.  see  Howard  and  Small,  Tr.  Cardiff  Nat.  Soc.  (1897)  xxviii,  part  i, 
with  plate. 


206  BRITISH  TACHYLYTES. 

In  America  ancient  olivine-basalts  have  been  described 
from  Notre  Dame  Bay  in  Newfoundland1,  North  Haven  in 
Maine2,  South  Mountain  in  Pennsylvania3,  the  Penokee 
(Huronian)  group4,  Keweenaw  Point,  etc.  (Mich.),  and  other 
localities  in  the  Lake  Superior  region5,  the  Grand  Canon  of 
the  Colorado6,  and  other  districts  of  pre-Cambrian  and  Lower 
Palaeozoic  rocks. 

The  name  tachylyte  is  commonly  employed  to  cover  the 
glassy  representatives  of  both  the  basalts  and  the  pyroxene- 
andesites.  Examples  occur  at  numerous  places  in  the  Tertiary 
volcanic  districts  of  Skye,  Raasay,  and  Mull7,  and  in  Co.  Down 
(Slievenalargy)8.  All  these  are  thin  selvages  of  dykes  and 
sills,  the  most  considerable  development  of  basic  glass  in 
Britain  being  at  a  locality  near  Loch  Scridain  in  Mull9.  The 
rocks  usually  enclose  small  crystals  of  magnetite  and  some- 
times of  olivine,  augite,  and  felspar.  The  glass  is  crowded 
with  incipient  growths  of  magnetite  and  occasionally  of  other 
minerals.  These  take  the  form  of  globulites,  sometimes 
collected  into  cumulites  (the  Beal  in  Skye),  of  margarites 
(Lamlash  near  Arran),  or  of  numerous  minute  opaque  rods 
(Some  in  Mull,  etc.),  sometimes  accompanied  by  transparent 
crystallites  and  belonites  (Gribun  in  Mull).  Spherulites  occur 
in  some  instances.  In  the  tachylyte  of  Ardtun  in  Mull10  they 
are  sometimes  isolated,  sometimes  in  bands,  sometimes  packed 
together,  with  polygonal  boundaries  to  the  exclusion  of  any 
glassy  matrix.  When  imperfect,  they  seem  to  consist  of  brown 
globulitic  matter,  which  is  more  condensed  towards  the 
centres.  When  better  developed,  they  shew  radiating  fibres 

1  Wadsworth,  A.  J.  S.  (1884)  xxviii,  95. 

2  G.  O.  Smith,  Joh.  Hopk.  Univ.  Circ.  No.  121  (1895). 

3  G.  H.  Williams,  A.  J.  S.  (1892)  xliv,  490-492. 

4  Van  Hise,  Monog.  xix  U.  S.  Geol.  Sur.  (1892)  410. 

5  Pumpelly  (Irving),  Copper-bearing  Rocks,  etc.,  Monog.  v  U.  S.  Geol. 
Sur.  (1884)  69-77,  pi.  ix. 

6  Iddings,  Uth  Ann.  Rep.  U.  S.  Geol.  Sur.  (1894)  520-524. 

7  Judd  and  Cole,  Q.  J.  G.  S.  (1883)  xxxix,  444-462,  pi.  xin,  xiv.    For 
localities  of  numerous  other  examples  in  Mull,  see  Kendall,  G.  M.  1888, 
555-560. 

8  Kutley,  Journ.  Roy.  Geol.  Soc.  Ire.  (1877)  iv,  227-232,  pi.  xiv. 

9  Heddle,  Tr.  G.  S.  Glasg.  (1895)  x,  81-85. 

10  Cole,  Qt  J.  G.  S.  (1888)  xliv,  300-307,  pi.  xi. 


BRITISH   VARIOLITES.  207 

arranged  in  sectors,  some  brown  and  others  grey,  with  pleo- 
chroism  in  both  cases.  But  little  is  known  of  tachylytes 
among  the  older  volcanic  rocks1. 

Closely  allied  to  the  spherulitic  tachylytes  are  the  rocks 
known  as  variolite,  of  which  examples  have  been  described 
from  Anglesey,  the  Lleyn  district  of  Caernarvonshire,  and 
various  parts  of  Ireland2.  The  spherules  shew  considerable 
variety  of  structure,  ranging  from  mere  fan-like  groupings  of 
felspar  microlites  (cf.  fig.  44  ^1)  or  sheaf-like  aggregates  with 
a  lath-shaped  crystal  as  nucleus  (see  Sollas)  to  very  regular, 
radiate,  spherulitic  growths.  They  may  be  closely  packed  to 
make  up  the  entire  mass  of  a  portion  of  the  rock,  or  arranged 
in  bands,  or  isolated  in  a  matrix  of  brown  or  greenish  glass 
with  cumulites,  globulites,  etc.  (see  Cole).  The  individual 
spherules  are  commonly  from  one-tenth  to  one-half  of  an  inch 
in  diameter,  but  sometimes  less  or  more.  Secondary  changes 
may  cause  devitrification  of  any  glassy  matrix,  and  give  rise 
to  a  separation  of  iron- oxides,  a  production  of  epidote,  etc. 
An  example  remarkable  alike  for  the  large  scale  of  its  structure 
and  the  perfection  of  its  preservation  comes  from  Camas 
Daraich  at  the  southern  extremity  of  Skye3.  Here  the 
spherules,  sometimes  as  much  as  2  or  3  inches  in  diameter, 
are  built  of  radiating  felspar  fibres  with  minute  skeleton 
crystals  of  olivine  and  granules  of  augite,  while  in  one  variety 
of  the  rock  there  is  a  considerable  amount  of  interstitial  glassy 
base.  Variolite  is  found  sometimes  in  small  dykes,  sometimes 
as  a  margin  to  larger  basic  intrusions  or  lava-flows,  sometimes 
again  in  the  interior  of  a  diabase- mass,  either  bordering 
spheroidal  joints  or  forming  a  selvage  on  irregular  pillow-like 
portions  into  which  the  rock-mass  is  divided4. 

1  See  Groom,  Q.  J.  G.  S.  (1889)  xlv,  298-304,  pi.  xn  (Carrock  Fell). 

2  Miss  Kaisin  (Lleyn),  Q.  J.  G.  S.  (1893)  xlix,  145-159,  pi.  i ;  Cole 
(Careg  Gwladys,  Anglesey),  Sci.  Proc.  Roy.  Dubl.  Soc.  (1891)  vii,  112-120, 
pi.  x ;  (Annalong,  Co.  Down)  ibid.  (1892)  511-519,  pi.  xxi ;  (Dunmore 
Head,  Co.  Down)  ibid.   (1894)  viii,  220-222;    Sollas  (Roundwood,  Co. 
Wicklow)  ibid.  (1893)  99-106,  figures.     For  coloured  figure  of  the  'vario- 
lite of  the  Durance '  see  Fouque  and  Levy,  pi.  xxiv,  fig.  2. 

3  Clough  and  Harker,  Tr.  Edin.  G.  S.  (1899)  vii,  381-389,  pi.  xxm. 

4  On   this   and  other  points   see   Cole  and  Gregory  (M.  Genevre), 
Q.  J.  G.  S.  (1890)  xlvi,  295-332,  pi.  xin ;  Gregory  (Fichtelgebirge),  ibid. 
(1891)  xlvii,  45-62. 


208  BRITISH   LIMBURGITES. 

Certain  lavas  of  very  restricted  distribution  verge  on  the 
ultrabasic  in  composition1.  Here  belong  the  limburgites  of 
Rosenbusch  (magma-basalts  of  Boricky),  lavas  of  highly  basic 
nature,  rich  in  olivine  and  augite  and  devoid  of  felspar.  The 
best  British  examples  yet  recorded  are  from  the  Carboniferous 
of  Scotland  and  Ireland.  Dr  Hatch2  has  described  one  from 
Whitelaw  Hill,  near  Haddington,  which  is  in  a  very  fresh 
condition.  There  are  abundant  well-shaped  phenocrysts  of 
olivine  and  augite,  the  latter  having  a  very  pale  violet- brown 
tint  in  the  interior,  deepening  towards  the  margin,  with  slight 
pleochroism.  These  minerals,  with  imperfect  crystals  of 
magnetite,  occur  in  a  ground-mass  consisting  of  small  augite- 
prisms  set  in  brown  to  pale  yellowish  or  colourless  glass 
(fig.  44,  B}.  Prof.  Watts3  has  noted  a  lirnburgite  in  the 
Limerick  district  (Nicker),  which  closely  resembles  the 
preceding,  though  less  perfectly  preserved,  the  olivine  being 
replaced  by  carbonates,  etc.  The  augite  has  a  strong  zonary 
structure,  the  violet-brown  tint  being  noticeable,  while  the 
interior  of  each  crystal  is  paler  or  has  a  greenish  colour. 
Augite  in  a  second  generation,  magnetite  granules,  and  more 
or  less  altered  glass  make  up  the  ground-mass.  Similar  lavas 
occur  at  Phillipstown  is  Queen's  County4,  and  Prof.  Watts  has 
also  detected  a  limburgite  among  the  probably  Tertiary 
volcanic  rocks  of  Scalnagowan  in  Clare.  These  British 
examples  are  sufficiently  like  the  typical  rocks  of  Limburg5, 
near  the  Kaiserstuhl,  etc.,  to  render  detailed  description  of 
these  unnecessary.  They  are  characteristically  very  basic 
lavas,  in  which  crystallization  has  been  arrested,  both  in  the 
' intratelluric '  arid  in  the  'effusive'  period,  before  the 
separation  of  felspar  had  begun.  The  olivine  is  often  a 
variety  rich  in  iron,  and  becomes  converted  at  the  margin  of 
the  crystal  into  deep  red  haematite  or  brown  limonite6. 


1  Prof.  Bonney  remarks  that  chemically  the  limburgites  occupy  a 
transitional  position  between  the  olivine-dolerites  and  the  picrites:  G.  M. 
1901,  411-417. 

2  Trans.  Roy.  Soc.  Edin.  (1892)  xxxvii,  116,  117,  pi.  i,  fig.  1. 

3  Rep.  Brit.  Ass.  for  1892,  727. 

4  Watts,  Guide,  38,  94. 

5  Cohen  (3),  pi.  xxni,  fig.  3 ;  xxv,  fig.  1  ;  LXI,  fig.  1. 

6  Fouque  and  Le"vy,  pi.  LIT,  fig.  2. 


HORNBLENDE-BASALTS :     ORTHOCLASE-BASALTS.      209 

The  hornblende-basalts,  in  which  brown  hornblende  occurs 
as  phenocrysts,  are  a  peculiar  group,  of  thoroughly  basic 
composition.  Examples  occur  in  the  Rhon  district  and  the 
Westerwald,  in  Madagascar1,  etc.  Basalts  in  which  hornblende 
is  a  prominent  ferro-magnesian  constituent  are,  however,  not 
unknown  in  the  Carboniferous  of  Britain  (Elie  in  Fife). 

A  peculiar  group  of  basic  lavas  rich  in  alkali  may  be 
termed  orthoclase-basalts,  this  mineral  figuring  largely  in  the 
ground-mass  of  the  rocks.  They  have  been  described  by 
Iddings2  as  dykes  and  flows  occurring  at  numerous  places 
in  the  Yellowstone  Park  district.  The  most  basic  varieties 
(Absaroka  type)  have  phenocrysts  of  olivine  and  augite  ;  in 
the  Shoshone  type  labradorite  comes  in  in  addition ;  and  in 
the  Banak  type,  including  the  most  acid  of  the  rocks,  this 
mineral  preponderates.  Here  too  the  rocks  become  more 
felspathic,  and  biotite  largely  replaces  augite.  Rocks  corre- 
sponding with  the  Absaroka  type  occur  in  the  Bozeman 
district,  Montana3.  Although  mentioned  in  this  place,  these 
alkali-basalts  recall  by  their  remarkable  association  of  minerals 
certain  rocks  (ciminites)  which  we  have  considered  as  basic 
trachytes. 

1  Hatch,  Q.  J.  G.  S.  (1889)  xlv,  349-352. 

3  Journ.  Geol.  (1895)  iii,  935-959. 

»  Merrill,  Proc.  U.  S.  Nat.  Mm.  (1895)  xvii,  638-641,  665-671. 


H.  P.  14 


CHAPTER  XV. 

LEUCITE-  AND   NEPHELINE-BASALTS,  ETC. 

WE  shall  group  together  for  convenience  various  basic  and 
ultrabasic  lavas  in  which  leucite,  nepheline,  or,  in  certain 
types,  melilite  is  a  prominent  constituent,  with  or  without  a 
lime-soda-felspar.  In  the  phonolites  and  leucitophyres,  de- 
scribed above,  a  potash-felspar  was  an  essential  mineral,  and 
the  rocks  had  other  affinities  with  the  trachytes.  Although 
some  of  the  rocks  to  be  noticed  resemble  the  phonolites  and 
leucitophyres  in  some  features,  they  are  for  the  most  part 
allied  rather  with  the  basalts,  while  the  varieties  having  any 
considerable  amount  of  glassy  base  graduate  into  the  limburg- 
ites  and  augitites1. 

The  rocks  in  which  leucite  or  nepheline  only  partly  takes 
the  place  of  felspar  are  termed  leucite-  or  nepheline-tephrites 
when  free  from  olivine,  and  leucite-  or  nepheline-basanites  when 
containing  that  mineral.  For  those  rocks  which  have  the 
felspathoid  minerals  to  the  exclusion  of  felspar  the  name 
leucitite  or  nephelinite  is  used  when  olivine  is  absent,  and 
leucite-  or  nepheline-basalt  when  olivine  is  present.  In  all 
these  divisions  the  leucite-bearing  and  the  nepheline-bearing 
types  are  on  the  whole  distinct,  though  the  rocks  characterized 
by  either  of  the  minerals  may  contain  the  other  as  an  ac- 
cessory. 

To  these  types  may  be  added  the  melilite-basalts,  in  which 
the  mineral  named  is  abundant,  usually  with  little  or  no 

1  See,  e.g.,  G.  H.  Williams,  A.  J.  8.  (1889)  xxxvii,  188  (Fernando  de 
Noronha). 


MINERALS   OF   LEUCITE-BASALTS,   ETC.  211 

felspar  and  with  abundant  olivine.  Rosenbusch  separates 
from  the  lavas,  under  the  name  alnoite,  a  rock  which  occurs 
in  dykes  in  association  with  nepheline-syenite,  and  has  affinities 
with  some  of  the  lamprophyres. 

The  rocks  here  noticed  are  known  chiefly  from  districts 
of  Tertiary  and  Recent  volcanic  rocks.  A  few  examples  of 
Palaeozoic  age  have,  however,  been  recorded :  leucite-tephrite 
from  the  Maconnais,  leucitite  from  Siberia,  melilite-basalt 
from  Canada,  etc. 

Constituent  minerals.  The  leucite  of  these  rocks  may 
be  in  two  generations,  differing  in  size.  The  crystals  are 
always  idiomorphic  icositetrahedra,  but  often  more  or  less 
rounded.  They  usually  shew  feeble  birefringence  and  the 
characteristic  lamellar  twinning1.  Augite  microlites  and 
granules,  glass-inclusions,  etc.,  are  often  arranged  in  zones, 
or  grouped  in  the  centre  of  the  crystal2. 

The  nepheline  in  the  porphyritic  types  is  usually  confined 
to  the  ground-mass.  In  the  nephelinites  and  nepheline-basalts 
it  is  commonly  idiomorphic,  except  in  some  of  the  holocrystal- 
line  rocks.  In  other  types  it  often  forms  small  allotrio- 
morphic  crystals,  not  easily  identified,  and  its  distribution 
may  be  local.  It  can  sometimes  be  made  evident  by  staining 
with  fuchsine.  The  common  alteration -products  are  natrolite 
and  other  soda-zeolites  in  radiating  aggregates. 

Other  felspathoid  minerals,  sodalite,  hauyne*,  and  nosean> 
are  not  uncommon  as  phenocrysts  in  the  rock-types  richest 
in  leucite  and  nepheline,  but  they  occur  only  as  accessories. 

The  yellow  or  colourless  melilite4  is  recognized  by  its  weak 
double  refraction,  straight  extinction,  and  peculiar  micro- 
structure.  Idiomorphic  crystals  have  a  tabular  habit  parallel 
to  the  base,  and  the  basal  faces  sometimes  form  concave  curves. 
The  mineral  may  also  be  quite  allotriomorphic,  and,  when  it 
occurs  as  an  accessory  in  leucite-lavas,  has  sometimes  the 

1  Cohen  (3),  pi.  xxvni,  fig.  3. 

2  Ibid.  pi.  vii,  fig.  1 ;  xiv,  fig.  1 ;  xvn,  fig.  2 ;  xix,  fig.  1. 

3  Ibid.  pi.  xxi,  fig.  3. 

4  For  good  figures  see   Stelzner,  Neu.  Jahrb.,  Beil.  Bd.   ii  (1882), 
pi.  vm. 

14—2 


212  MINERALS   OF   LEUCITE-BASALTS,   ETC. 

form   of  a  framework  enclosing  other  minerals  in  poecilitic 
fashion  (fig.  48). 

This  latter  mode  of  occurrence  is  sometimes  seen  also  in 
the  sanidine  which  occurs  as  an  accessory  in  some  of  the 
leucite-  and  nepheline-lavas,  Uniting  them  with  the  leucito- 
phyres  and  phonolites.  The  plagioclase  felspars,  which  are 
found  in  some  types  of  these  rocks,  are  always  of  a  basic 
variety.  There  may  be  phenocrysts  with  idiomorphic  outline, 
tabular  habit,  albite-lamellation,  zonary  structure,  and  zones 
of  glass-inclusions;  while  the  felspars  of  the  ground-mass  vary 
from  narrow  laths,  often  only  once  twinned,  to  mere  micro- 
lites.  These  shew  a  tendency  to  spherulitic  arrangement,  and 
the  phenocrysts  too  may  form  radially  grouped  aggregates 
(fig.  47). 

The  usual  coloured  constituent  in  the  rocks  here  considered 
is  augite.  It  often  occurs  in  two  generations,  the  earlier 
relatively  large  and  well  shaped1.  The  colour  is  commonly 
green,  but  often  varies  in  concentric  zones2,  becoming  some- 
times pale  violet,  with  distinct  pleochroism,  at  the  margin 
of  a  crystal.  Again,  there  are  sometimes  two  kinds  of  por- 
phyritic  augite,  differently  coloured.  Some  nephelinites  have 
a  purple-brown,  pleochroic,  'hour-glass'  augite.  Exceptionally 
some  of  the  rocks  contain  little  yellowish-green  needles  of 
wgirine.  A  brown  or  red-brown  or  red  biotite  is  very  common 
in  the  nepheline-  and  melilite-rocks,  often  shewing  resorption- 
phenomena.  Brown  hornblende  is  an  occasional  accessory  in 
some  rocks,  and  commonly  shews  a  corrosion- border  of  mag- 
netite and  augite3. 

Olivine  is  an  essential  constituent  in  many  of  the  types, 
and  has  the  same  general  characters  as  in  basalts.  In  some 
of  the  most  basic  rocks  the  mineral  is  a  hyalosiderite,  and 
often  becomes  red  by  the  separation  of  iron- oxide. 

Iron-ores  are  commonly  present,  and  in  the  olivine-bearing 
rocks  often  abundant.  They  are  magnetite  and  ilmenite,  the 
latter  sometimes  in  deep  brown  translucent  scales. 

1  Cohen  (3),  pi.  XLII,  figs.  1,  2. 

2  Ibid.  pi.  n,  fig.  4 ;  xvm,  fig.  4. 

3  Ibid.  pi.  iv,  fig,  4. 


LEUCITE-TEPHRITES.  213 

Apatite  is  a  pretty  constant  accessory,  usually  in  little 
prisms  with  the  characteristic  cross -jointing1,  though  in  some 
of  the  nepheline-dolerites,  etc.,  it  builds  larger  and  stouter 
crystals.  A  pale  violet  or  blue  tint,  with  evident  dichroism, 
is  not  infrequent.  Some  of  the  leucite-  and  nepheline- lavas 
have  melanite-g&YUGt,  brown  in  slices  and  always  iso tropic. 
A  very  common  accessory  in  the  melilite -basalts  and  some 
nepheline-rocks  is  perofskite  in  minute  octahedra,  shewing  in 
high  relief  in  consequence  of  their  refractive  index2. 

Leading  types.  Our  illustrations  must  be  drawn  almost 
entirely  from  foreign  sources,  since,  with  the  exception  of  the 
few  phonolites  already  noted,  lavas  containing  felspathoid 
minerals  are  not  found  within  the  British  area. 

It  must  be  noticed  that  the  several  types  to  be  dis- 
tinguished are  not  always  sharply  marked  off  from  one 
another.  This  is  especially  the  case  with  the  felspar-bearing 
members,  the  tephrites  and  the  basanites  having  in  great 
measure  the  same  general  characteristics,  except  for  the  not 
very  considerable  proportion  of  olivine  in  the  latter.  The 
differences  between  the  leucitites  and  nephelinites  on  the  one 
hand  and  the  leucite-  and  nepheline -basalts  on  the  other  are, 
however,  more  marked,  the  olivine-bearing  types  being  notably 
richer  in  the  ferro-magnesian  constituent  (augite)  and  in  iron- 
ores.  Among  rocks  characterized  specially  by  melilite,  the 
only  important  type  is  melilite -basalt,  containing  abundant 
olivine  and  typically  no  felspar. 

A  well-known  leucite-tephrite  comes  from  Tavolato3  near 
Rome.  It  is  remarkable  for  an  abundance  of  blue  haiiyne. 
There  are  two  generations  of  leucite,  both  shewing  twin-lamel- 
lation.  A  greenish-brown  segirine  occurs  as  well  as  augite. 
Both  lath-shaped  plagioclase  and  sanidine  are  found,  the  latter 
sometimes  occurring  as  an  interstitial  matrix  to  the  other 
minerals,  though  in  other  examples  there  is  some  glassy 
residue.  The  rock  also  contains  grains  of  melanite.  Other 


1  Cohen  (3),  pi.  XLVII,  fig.  1. 

2  Ibid.  pi.  in,  fig.  1. 

3  Ibid.  pi.  xxvii,  fig.  2. 


214  LEUCITE-BASANITES. 

Italian  examples  have  been  described1,  and  leucite-tephrites 
have  also  been  described  from  the  Kaiserstuhl  (near  Freiburg 
in  the  Breisgau),  from  Bohemia,  etc.  The  Bohemian  rocks 
contain  no  haiiyne,  and  have  leucite  confined  chiefly  to  the 
holocrystalline  ground-mass. 

The  lavas  of  Vesuvius2  stand  between  leucite- tephrite  and 
leucite-basanite,  olivine  being,  as  a  rule,  not  very  abundant. 
The  conspicuous  phenocrysts  are  of  leucite  (with  inclusions  of 
brown  glass  and  augite-microlites),  plagioclase  (often  in  radi- 
ating groups  of  crystals),  augite,  and  usually  olivine  (fig.  47), 
and  the  same  minerals,  except  the  last,  recur  as  constituents 
of  the  ground-mass.  Magnetite  and  apatite  are  always  present, 
and  in  some  cases  biotite  is  plentiful.  Nepheline,  sanidine, 
and  brown  hornblende  are  rarer,  and  sodalite  is  confined  to 
crevices,  where  it  seems  to  have  been  formed  after  the  con- 
solidation of  the  rock.  The  ground-mass  is  usually  holo- 
crystalline or  with  only  a  little  brownish  or  yellowish  glass, 
but  there  are  vitreous3  and  pumiceous  modifications.  The 
lavas  of  Vulcanello,  as  described  by  Backstrom,  represent  a 
different  variety,  also  poor  in  olivine.  Leucite  is  confined  to 
the  ground-mass,  and  part  of  the  felspar  in  the  ground  is  of  a 
potash-bearing  species. 

The  rock  described  by  Hague4  from  the  Absaroka  range 
in  Wyoming  resembles  a  leucite-basanite,  but  has  affinities 
with  the  leucitophyres.  Olivine  and  augite  are  porphyritic  in 
a  ground-mass  essentially  of  leucite  and  sanidine,  plagioclase 
being  only  scantily  represented.  Magnetite,  apatite,  and  a 
little  mica  are  present,  and  there  may  be  a  very  small 
proportion  of  glassy  base. 

The  scoriaceous  lava  of  Niedermendig,  in  the  Laacher  See 
district,  which  has  been  largely  employed  for  mill-stones5,  is 

1  Washington,  Journ.  Geol  (1896)  iv,  561-564  (Bolsena) ;  ibid.  (1897) 
v,  42,  43  (L.  Bracciauo),  and  246-248  (Rocca  Monfina). 

2  Cohen  (3),  pi.  n,  fig.  4 ;  xiv,  fig.  1 ;  xvn,  fig.  2 ;  xix,  fig.  1 ;  xxxix. 
fig.  4 ;  Fouque  and  Le>y,  pi.  XLIX,  fig.  1 ;  Haughton  and  Hull,  2V.  Roy. 
Ir.  Acad.  (1875)  xxvi,  pi.  n. 

3  Fouqu6  and  Le'vy,  pi.  XLI,  fig.  2 ;  Cohen  (3),  pi.  in,  fig.  2. 

4  A.  J.  S.   (1889)   xxxviii,  45.     This  rock   falls   under  the  leucite- 
absarokite  of  Iddings,  Journ.  Geol.  (1895)  iii,  939. 

5  Cf.  Clements,  Bull.  No.  5  Geol.  Sur.  Ala.  (1896)  142,  143. 


NEPHELINE-TEPHRITES.  215 

placed  between  leucite-  and  nepheline-tephrite.  Its  conspi- 
cuous crystals  of  haiiyne  are  regarded  by  Lehman  n  as  of 
foreign  derivation.  The  so-called  haiiynophyre  of  Mte  Vulture, 
near  Melfi,  has  both  leucite  and  nepheline  in  its  ground-mass, 
while  the  most  abundant  phenocrysts  are  of  blue  haiiyne  and 
yellow  augite. 


ol 


FIG.  47.     LEUCTTE-BASANITE,  VESUVIUS  ;    x  20. 

This  shews  leucite  (I)  and  crystals  or  groups  of  felspar  (/),  both  with 
zones  of  inclusions,  augite  (aw),  olivine  (oi),  magnetite,  and  a  little 
isotropic  residue  [845]. 

The  lavas  of  the  Canary  Islands  afford  a  great  variety  of 
nepheline-tephrites1  and  nepheline-basanites,  the  former  pre- 
dominating. Some  of  them  are  of  the  so  called  'basaltoid' 
type,  in  which  nepheline  is  not  present  in  any  large 
proportion.  The  structure  is  usually  holocrystalline.  The 
*  phonolitoid '  type  is  richer  in  nepheline,  and  sometimes  has 
blue  or  yellow  haiiyne.  Here  hornblende  is  found  in  varying 
proportion,  sphene  occurs,  and  a  predominance  of  sanidine 
over  plagioclase  in  some  varieties  indicates  affinity  with  the 
phonolites. 

1  Cohen  (3),  pi.  iv,  fig.  4 ;  xvi,  fig.  3  ;  LXVI,  figs.  3,  4. 


216  NEPHELINE-BASANITES. 

Hornblende-bearing  nepheline-tephrites  occur  also  in  the 
Rhon  (to  the  north  of  Bavaria),  in  the  Thiiringer  Wald,  etc. 
There  are  also  rocks,  named  '  basanitoid '  by  Bucking,  having 
no  actual  nepheline,  but  a  glassy  base  very  rich  in  soda  to 
represent  that  mineral. 

Nepheline-tephrites  have  been  described  by  ZirkeP  from 
the  Kawsoh  Mts  in  Nevada.  These  have  sanidine  predomin- 
ating over  the  plagioclase :  augite  crystals  and  needles,  mag- 
netite, and  interstitial  nepheline  are  the  other  constituents. 
From  the  Elkhead  Mts  and  other  localities  in  Colorado  the 
same  writer2  notes  examples  of  nepheline-basanite.  One  type, 
of  coarse  texture,  has  large  crystals  of  olivine,  idiomorphic 
zoned  augite,  plagioclase,  and  interstitial  nepheline.  Magnetite 
is  plentiful,  and  biotite  is  often  present.  A  nepheline-basanite 
from  Southern  Texas3,  on  the  other  hand,  is  of  a  type  poor  in 
olivine,  carrying  brown  hornblende  among  the  phenocrysts  and 
sanidine  in  the  ground-mass.  From  the  western  (Trans -Pecos) 
district  of  Texas  comes  a  nepheline-tephrite  containing  abun- 
dant green  augite,  brown  hornblende,  and  biotite  in  a  ground- 
mass  of  plagioclase  and  nepheline4. 

Nepheline-basanites  in  considerable  variety  are  associated 
with  the  nepheline-tephrites  of  the  Rhon,  the  Canaries,  etc. 
Some  are  poor  in  nepheline  and  felspar,  and  approximate  to 
the  limburgites.  Doelter's  'pyroxenite'  (augitite)  from  the 
Cape  Verde  Islands  is  similar,  having  only  crystals  of  augite 
and  some  magnetite  in  a  glassy  ground-mass  of  composition 
agreeing  with  nepheline. 

Good  examples  of  the  type  leucitite  come  from  the  Alban 
Hills,  near  Rome  (Capo  di  Bove5,  etc.}.  They  are  non- 
porphyritic  rocks,  very  rich  in  leucite  and  relatively  poor  in 
augite.  Other  constituents  are  brown  biotite,  yellow  striated 
melilite,  and  clear  sanidine,  all  of  which  occur  in  crystal-plates 

1  Micro.  Petrogr.  Fortieth  Parallel  (1876),  255,  256. 
a  Ibid.  256-258. 

3  Osann,  Journ.  Geol.  (1893)  i,  344-346. 

4  Osann,  4£/i  Ann.  Rep.  Geol.  Sur.  Tex.  (1892)  134. 

5  Cohen  (3),  pi.  n,  fig.  2,  and  pi.  xxvm,  fig.  3;  Fouque  and  Le"vy, 
pi.  L,  fig.  1.     See  also  fig.  2  of  latter  for  a  type  richer  in  augite,  from 
Frascati. 


LEUCITITES.  217 

enclosing  the  leucite  and  augite  in  poecilitic  or  ophitic  fashion 
(fig.  48).  Other  leucitites  come  from  neighbouring  volcanic 
districts l. 


FIG.  48.     LEUCITITE,  CAPO  DI  BOVE,  NEAR  KOME  ;    x  100. 
Small  leucites  with  zonally  grouped  inclusions  are  numerous,  and 
augite  and  magnetite  also  occur.   All  these  are  enclosed  by  a  large  crystal 
of  yellowish  striated  melilite.     In  other  parts  of  the  slide  sanidine  plays 
a  similar  part  [G  243]. 

The  rock  described  by  Zirkel2  from  the  Leucite  Hills, 
Wyoming,  is  even  richer  in  leucite.  In  addition  to  this 
mineral,  it  contains  only  a  pale  biotite,  scattered  needles  of 
green  augite,  apatite,  and  a  small  quantity  of  magnetite. 
Kemp3  has  shewn,  however,  that  the  lavas  forming  these 
hills  present  considerable  variation.  In  particular  the  leucite 
gives  place  to  sanidine  in  various  proportions,  affording  trans- 
itions to  leucitophyre.  From  the  variety  containing  leucite 
to  the  exclusion  of  sanidine  (Wyoming  type)  Cross4  has 
separated  that  in  which  both  minerals  are  well  represented 
(Orenda  type).  He  has  described  also  another  rock  consisting 

1  Washington,  Journ.  Geol.  (1896)  iv,  556-558  (Bolsena);  ibid.  (1897) 
v,  41,  42  (L.  Bracciano),  46,  47  (Cerveteri),  245  (Kocca  Monfina). 

2  Micro.  Petrogr.  Fortieth  Parallel,  260,  261 ;   pi.  v,  fig.  4 ;   i,  figs. 
21-23. 

3  Bull.  G.  S.  Amer.  (1897)  viii,  175-180. 

4  A.  J.  S.  (1897)  iv,  120-133,  and  in  Diller,  186-191. 


218  LEUCITE-BASALTS. 

of  diopside  and  yellow  mica  with  a  glassy  base  which  has  the 
composition  of  leucite  (Madupa  type). 

A  leucitite  from  the  Bear-paw  Mts  of  Montana1  contains 
phenocrysts  of  augite  and  leucite  in  a  ground-mass  consisting 
essentially  of  minute  skeleton  leucites  with  very  little  inter- 
stitial glass. 

Of  leucite-basalt  good  examples  come  from  the  Eifel  district 
(Fornicher  Kopf,  Hummerich,  etc.).  These  have  phenocrysts  of 
olivine,  augite,  and  often  biotite,  in  a  ground-mass  which  is 
always  very  fine-grained  but  rarely  contains  any  glassy  residue. 
It  consists  of  predominating  augite  with  leucite  and  often 
nepheline,  while  a  little  sanidine  sometimes  occurs  interstitially. 

Weed  and  Pirsson2  have  described  specimens  from  the 
Bear- paw  Mts,  Montana.  Here  the  leucites,  up  to  -f^  inch  in 
diameter,  are  turbid  from  alteration.  The  other  phenocrysts 
are  olivine  and  pale  brown  zoned  augite,  and  these  minerals 
occur  abundantly  in  a  ground-mass  of  magnetite  grains, 
augite  microlites,  and  what  appears  to  be  a  colourless  glass. 

Leucite-basalt  has  been  described  from  localities  in  New 
South  Wales3.  The  abundant  olivine  has  a  somewhat 
peculiar  character.  This,  with  leucite  and  large  ragged  flakes 
of  yellow  mica,  belongs  to  the  earlier  stage  of  consolidation, 
while  the  ground-mass  of  the  rock  is  a  finely-crystalline 
aggregate  of  leucite,  yellowish-green  augite,  and  magnetite, 
with  occasionally  a  little  glass. 

The  rocks  rich  in  nepheline  are  almost  always  holocrystal- 
line.  A  well-marked  type  is  the  doleritic  nephelinite  or 
nepheline-dolerite  of  Lobau  in  Saxony,  a  rock  of  comparatively 
coarse  texture,  with  abundant  nepheline.  The  augite  is  of  a 
purple-brown  pleochroic  variety,  with  hour-glass  or  other  zonary 
growth,  and  often  idiomorphic  (fig.  49).  Locally  the  structure 
of  the  rock  may  become  intersertal  or,  again,  micrographic4. 
Besides  the  abundant  nepheline,  subordinate  sanidine  may 

1  Weed  and  Pirsson,  A.  J.  S.  (1896)  ii,  144-148,  with  figures. 

2  A.  J.  S.  (1896)  i,  288-290. 

3  Judd,  M.  M.  (1887)  vii,  194,  195 ;  Edgeworth  David  and  Anderson, 
Eec.  Geol.  Sur.  N.  S.  W.  (1890)  i,  159-162,  pi.  xxvni. 

4  Cohen  (3),  pi.  xxxiv,  fig.  2. 


NEPHELINITES. 


219 


occur,  and  more  rarely  a  plagioclase.  The  common  iron-ore 
is  a  titaniferous  magnetite,  and  apatite  needles  occur  abund- 
antly. In  the  otherwise  similar  type  of  Meiches,  in  the 
Vogelsberg  (Hesse),  leucite,  in  irregular  grains  crowded  with 
apatite  needles,  becomes  a  prominent  constituent.  Both  rocks 
shew  transitions  to  nepheline-basalt,  of  finer  texture,  with  less 


FIG.  49. 


NEPHELINITE  (NEPHEHNE-DOLERITE),  LOBAURR  BERG, 
SAXONY  ;    x  20. 


The  minerals  shewn  are  nepheline  («),  some  felspar  (/),  purplish- 
brown  augite  (a)  with  hour-glass  structure,  magnetite  (m),  and  apatite 
(ap),  the  rock  being  holocrystalline.  The  coming  in  of  felspar  marks  a 
transition  to  the  tephrite  type  [G  220]. 

nepheline  and  with  abundant  phenocrysts  of  olivine.  The 
same  is  true  of  another  well-known  nephelinite,  that  of  Katzen- 
buckel  in  the  Odenwald  (Baden).  A  typical  nepheline  dolerite 
has  been  recorded  from  Shannon  Tier  in  Tasmania1. 

Another  type  ('basaltic  nephelinite')  occurs  in  the  Grand 
Canary,  etc.,  and  by  the  coming  in  of  plagioclase  passes  into 
the  tephrites.  It  is  of  fine  texture  and  much  richer  in  augite 

1  Twelvetrees  and  Petterd,  Papers  and  Proc.  JR.  S.  Tas.  for  1898-9 
(1900),  60-64. 


220  NEPHELINE-BASALTS. 

than  the  preceding.  Varieties,  some  rich  in  haiiyne,  occur  in 
the  Erzgebirge  and  in  Bohemia.  Rosenbusch's  'phonolitic' 
type  is,  on  the  other  hand,  poor  in  coloured  minerals,  and 
carries  no  augite-phenocrysts.  In  the  frequent  presence  of 
segirine-microlites,  the  abundance  of  idiomorphic  nepheline, 
and  the  coming  in  of  sanidine,  this  type  approaches  the 
phonolites. 

The  nepkeline-basalts,  much  more  widely  distributed  than 
nephelinites,  shew  less  variety  of  character.  They  are  typically 
holocrystalline  rocks  composed  of  nepheline,  augite,  and  olivine, 
with  some  magnetite  and  apatite.  Some  contain  biotite  in 
addition  to  augite,  and  haiiyne  may  accompany  the  nepheline  \ 
Such  rocks  are  known  in  Hesse  and  Thuringia,  the  Eifel,  many 
parts  of  Saxony,  Bohemia,  the  Cape  Verde  Islands,  Brazil2, 
and  other  districts  of  Tertiary  and  Recent  volcanic  rocks. 
The  chief  variation  depends  upon  the  coming  in  of  melilite 
in  addition  to  nepheline  (e.g.  Herchenberg  and  Bongsberg  in 
the  Eifel,  several  Saxon  localities,  etc.).  Leucite  is  a  less 
common  accessory. 

Several  American  examples  have  been  described.  From 
the  Cripple  Creek  district,  Colorado,  Cross3  notes  a  dyke  very 
rich  in  olivine,  augite,  and  magnetite,  with  a  subordinate 
colourless  base,  chiefly  of  nepheline.  From  southern  Texas 
Osann4  describes  a  rock  in  which  large  oli vines  are  abundant, 
with  magnetite  and  small  octahedra  and  grains  of  brownish- 
violet  perofskite.  The  holocrystalline  ground-mass  consists 
of  abundant  augite-prisms,  tabular  crystals  of  faint  yellow 
melilite  with  characteristic  cross-fibration  and  '  peg-structure ' 
(Ger.  Pflockstructur),  and  aggregates  of  shapeless  grains  of 
nepheline.  Felspar  is  entirely  wanting.  This  rock  is  inter- 
mediate between  nepheline-basalt  and  melilite-basalt. 

In  the  true  melilite-basalts  nepheline  is  wanting  or  at  most 
an  accessory.  Phenocrysts  of  olivine,  augite,  and  biotite  are 
embedded  in  a  usually  holocrystalline  ground-mass  of  smaller 

1  Fouqu6  and  Le'vy,  pi.  XLIX,  fig.  2. 

2  G.    H.   Williams,   A.   J.  S.  (1889)  xxxvii,  186,  187  (Fernando  de 
Noronha). 

3  16th  Ann.  Rep.  U.  S.  G.  S.  (1895)  part  n,  49,  50. 

4  Journ.  Geol.  (1893)  i,  341-343. 


MELILITE-BASALTS.  221 

biotite,  zoned  augite,  sometimes  olivine,  and  melilite.  The 
last  sometimes  occurs  also  among  the  phenocrysts.  Biotite  is 
specially  characteristic,  and  in  the  first  generation  may  form 
quite  large  flakes.  Rocks  answering  to  this  description  are 
known  from  Hochbohl,  near  Owen,  and  Urach,  in  Wurtemberg : 
from  Gorlitz,  in  the  Prussian  Lausitz ;  as  dykes  on  Alno,  an 
island  off  the  coast  of  Sweden ;  etc.  A  good  example,  of 
Silurian  age,  is  described  from  Ste  Aune  de  Bellevue,  near 
Montreal1.  Here  the  phenocrysts  are  brown  mica  in  large  and 
abundant  crystals,  olivine  more  or  less  converted  to  hematite, 
and  augite :  the  ground-mass  is  of  mica,  olivine,  augite, 
magnetite,  and  melilite,  with  apatite  and  perofskite,  the  last 
a  mineral  rarely  absent  from  such  rocks.  The  melilite  is  the 
latest  product  of  consolidation,  forming  imperfect  crystals  of 
tabular  habit  with  the  characteristic  *  peg-structure/  A  rock 
from  Mannheim,  N.Y.2,  differs  from  this  chiefly  in  the  absence 
of  pyroxene,  and  both  closely  resemble  the  typical  '  alnoite ' 
of  Alno,  off  the  coast  of  Sweden,  which  also  contains  augite 
in  addition  to  the  large  phenocrysts  of  brown  mica.  A  good 
melilite-basalt  has  been  described  from  Shannon  Tier  in 
Tasmania3. 

The  only  rocks  of  this  kind  known  in  Britain  are  those 
described  by  Dr  Flett4  from  the  Orkneys,  where  they  form 
dykes  cutting  the  Old  Red  Sandstone  and  associated  with 
others  of  camptonite  and  monchiquite.  One  from  Rennibuster, 
near  Kirkwall,  has  for  phenocrysts  large  irregular  plates  of 
biotite,  small  serpentinized  olivines,  and  some  large  idio- 
morphic  crystals  of  augite.  The  ground-mass  consists  of 
abundant  small  augites  of  purplish-brown  colour,  idiomorphic 
melilite,  and  interstitial  matter  representing  altered  glass  or 
perhaps  nepheline.  Another,  from  Naversdale  near  Orphir, 
has  the  melilite  in  allotriomorphic  patches,  shewing  peg- 
structure.  The  mineral  is  often  replaced  by  zeolites,  calcite, 
etc.  An  allied  type  is  described  under  the  name  melilite- 
inonchiquite. 

1  Adams,  A.  J.  S.  (1892)  xliii,  269-279. 

2  Smyth,  A.J.8.  (1893)  xlvi,  104-107;  (1896)  ii,  290-292. 

3  Twelvetrees  and  Petterd,  Papers  and  Proc.  E.  S.  Tas.  for  1898-9 
(1900)  60-64. 

4  Tr.  Roy.  Soc.  Edin.  (1900)  xxxix,  892-898,  pi.  m,  figs,  4-6, 


D.     SEDIMENTARY  KOCKS. 


UNDER  the  head  of  sedimentary  rocks  we  shall  include  the 
stratified  deposits  formed  for  the  most  part,  though  not  exclus- 
ively, under  water  by  the  accumulation  of  detritus  and  of 
fragmental  material  of  volcanic  origin,  by  organic  agency,  and 
by  chemical  action  or  the  evaporation  of  saline  solutions. 
The  last  clause  includes  the  secondary  cementing  material  of 
many  fragmental  rocks,  as  well  as  the  less  common  deposits  of 
rock-salt,  etc.,  which  do  not  demand  special  notice. 

The  rocks  exhibit  great  variety  of  composition  and  charact- 
ers, and  in  the  nature  of  the  case  do  not  admit  of  any  very 
strict  petrological  classification.  They  will  be  treated  mainly 
under  four  groups  :  the  coarser  detrital  deposits  (arenaceous), 
the  finer  detrital  deposits  (argillaceous),  the  rocks  consisting 
essentially  of  carbonate  of  lime  (calcareous),  and  the  fragmental 
volcanic  rocks  (pyroclastic  of  some  authors).  In  all,  with  the 
exception  of  some  of  the  calcareous  rocks,  a  fragmental  or 
'clastic'  structure  is  essentially- present :  this,  with  the  bedded 
occurrence,  may  be  taken  as  characteristic  of  the  whole. 


CHAPTER  XVI. 

ARENACEOUS   ROCKS. 

THE  arenaceous  rocks  are  typical  fragmented  ('clastic') 
accumulations,  consisting  of  grains  of  one  or  more  materials 
mechanically  derived,  to  which  may  be  added  interstitial 
matter  deposited  in  place.  There  is  thus  a  distinction  be- 
tween original  or  '  allothigenous '  constituents,  derived  from  a 
distance,  and  secondary  or  *  authigenous '  constituents  formed 
after  the  accumulation  of  the  grains.  The  fragmental  nature 
of  the  rocks  is  usually  evident  to  the  eye,  and  the  conditions 
of  deposition  in  water  may  be  indicated  by  an  appearance 
of  lamination,  but  this  is  rarely  so  well  marked  as  in  some 
argillaceous  rocks. 

The  name  sand  (Fr.  sable)  is  reserved  for  incoherent 
deposits  :  when  compacted  by  some  cementing  medium,  they 
become  sandstone  or  grit.  These  last  two  words  are  often 
used  synonymously,  though  different  writers  have  employed 
them  to  mark  various  distinctions.  If  a  distinction  be  made, 
it  is  perhaps  best  to  name  the  round-grained  rocks  sandstones, 
and  those  with  angular  grains  grits.  Such  epithets  as  felspathic 
and  calcareous  are  used  to  describe  the  nature  sometimes  of 
the  grains,  sometimes  of  the  cement :  they  usually  need  no 
explanation.  The  old  term  greywacke  (Ger.  Grauwacke)  has 
been  revived  for  a  complex  rock  with  grains  of  quartz,  felspar, 
and  other  minerals  and  rocks,  united  by  a  cement  usually 
siliceous.  An  arkose  is  a  deposit  derived  directly  from  the 
destruction  of  granite  or  gneiss,  and  containing  abundant 
felspar.  A  quartzite  (of  the  type  belonging  here)  is  a  rock 
consisting  of  grains  chiefly  of  quartz  with  a  quartz  cement. 


224  MINERALS   OF   SAND-GRAINS. 

The  coarsest  clastic  deposits,  in  which  pebbles  occur  as 
well  as  grains,  are  named  conglomerate  or  pudding-stone  (Fr. 
poudingue)  when  the  large  fragments  are  rounded,  and  breccia 
(Fr.  breche)  when  they  are  angular.  These  rocks  will  require 
but  little  notice. 

Derived  grains1.  Since  most  sands  are  derived  directly 
or  indirectly  (i.e.  through  the  medium  of  earlier  sedimentary 
deposits)  from  the  waste  of  igneous  or  crystalline  rocks,  the 
most  usual  minerals  in  sand-grains  are  those  which  figure 
largely  in  the  composition  of  large  areas  of  rock,  such  as 
granites,  gneisses,  and  crystalline  schists.  But  chemical  pro- 
cesses tend  to  make  a  selection  among  these  constituents ;  for 
the  material  is  commonly  affected  by  partial  decomposition, 
either  prior  to  the  disintegration  of  the  parent  rock-masses, 
during  transport,  or  subsequently  to  the  accumulation  of  the 
clastic  deposit.  So  the  commonest  constituents  of  sands  are 
those  abundant  rock-forming  minerals  which  are  least  prone 
to  chemical  changes,  such  as  quartz  and  white  mica.  Felspars, 
augite,  hornblende,  and  dark  micas  may  occur  plentifully  in 
particular  deposits,  but  are  less  characteristic  of  sands  in 
general,  while  unstable  minerals  like  olivine  rarely  occur 
among  detrital  material.  Certain  accessories,  such  as  zircon 
and  rutile,  are  widely  distributed  in  sands,  but  only  in  small 
quantity.  Others  may  be  abundant  locally,  just  as  the  modern 
sands  on  our  coasts  are  found  in  particular  localities  to  be 
rich  in  garnet,  or  flint,  or  tourmaline,  or  ilmenite  (menac- 
canite) 2.  The  admixture  of  few  or  many  constituents  depends 
on  the  extent  and  geological  diversity  of  the  drainage-area 
from  which  the  material  was  derived.  River-  and  lake-sands 
usually  shew  less  variety  than  those  of  marine  origin3. 

1  For  much  information  on  sand-grains  see  Sorby,  Presid.  Address, 
Q.  J.  G.  S.  (1880)  xxxvi,  Proc.  pp.  47-65  ;  also  Annlv.  Address  Micro. 
Soc.  (1877)  Monthly  Micro.  Journ. 

2  The   heavier   accessories  may  be  separated  from  loose  sands  by 
levigation  in  water,  as  described  by  Mr  Dick,  A  New  Form  of  Polarising 
Microscope  (1890),  41-45.     A  useful   adjunct  for  this  purpose  is  the 
'batda'  or  Brazilian  miner's  pan  :  see  Derby,  Proc.  Rochester  Acad.  Sci. 
(1891)  i,  198-206.     For  a  dry  method,  see  Carus- Wilson,  Nature  (1889), 
xxxix,  591.     For  an  example  of  a  systematic  investigation,  see  Retgers 
on  the  dune-sands  of  Holland,  M.  M.  xi,  113,  114  (Abstr.). 

a  See  Julien  and  Bolton,  Proc.  Amer,  Assoc.  (1884)  413-416, 


FORMS   OF   SAND-GRAINS.  225 

Some  coarse-grained  deposits  contain  composite  rock-frag- 
ments, e.g.,  a  piece  consisting  of  quartz  and  felspar  with  the 
relations  characteristic  of  granite.  Other  sandstones  have 
numerous  fragments  of  lava.  Recent  deposits  near  the  volcanic 
islands  of  the  Pacific  sometimes  consist  wholly  of  rolled  frag- 
ments of  lava,  pieces  of  decomposing  volcanic  glass  (palagouite), 
small  chips  of  pumice,  etc.  By  admixture  of  material  of 
directly  volcanic  origin  these  volcanic  sands  graduate  into  tufts. 

The  accumulations  composed  mainly  or  entirely  of  organic 
fragments  (shell-sands,  coral-sands,  etc!)  are  more  conveniently 
placed  with  the  limestones. 

The  form  and  superficial  cjutracters  of  sand-grains,  best 
studied  by  mounting  the  material  dry  or  in  water,  may  depend 
upon  the  properties  of  the  individual  minerals  and  their  mode 
of  occurrence  in  the  parent- rocks;  upon  the  effects  of  attrition 
during  transport ;  and  sometimes  upon  crystalline  growth 
subsequent  to  the  accumulation  of  the  deposit.  Grains  of 
felspar,  hornblende,  etc.,  usually  have  their  boundaries  partly 
determined  by  the  cleavages  of  the  mineral ;  mica  tends  to 
form  flat  flakes  or  scales;  minerals  like  zircon  and  anatase, 
which  in  the  parent-rock  built  small  well-formed  individuals, 
often  preserve  their  form  intact.  They  are  probably  released 
in  some  cases  by  the  destruction  in  the  sand  itself  of  an 
enclosing  mineral,  such  as  biotite.  Quartz  breaks  into 
fragments  of  irregularly  angular  outline.  If  originally  of 
interstitial  occurrence  (e.g.  in  a  granite)  it  partly  retains  its 
highly  irregular  contour,  and  the  minor  irregularities  produce 
a  rather  opaque  appearance  on  the  surface.  Quartz-grains 
from  a  fine  mica- schist,  on  the  other  hand,  tend  to  flaky  and 
lenticular  shapes. 

The  degree  of  rounding  produced  by  attrition  during  trans- 
port depends  on  the  hardness  of  the  mineral,  but  also  on  the 
nature  and  duration  of  the  mechanical  agencies  involved. 
Large  grains  are  often  more  rounded  than  small  (fig.  50). 
Marine  sands  are  in  general  more  round-grained  than  those 
of  rivers  and  lakes,  while  wind-borne  sands,  such  as  those 
of  deserts,  are  still  more  rounded  by  friction1.  Only  in 

1  For  illustrations  see  Tr.  Edin.  G.  S.  (1897)  vii,  pi.  xi,  fig.  1 ;  xix, 
figs.  2,  3. 

ii.  P.  15 


226 


SOURCES  OF  SAND-GRAINS. 


these  last  are  the   smallest   grains   ever   found   to   be   well 
rounded. 

It  is  usually  possible  to  form  some  opinion  as  to  the  source 
or  sources  of  the  derived  material  of  a  sand.  The  minerals 
identified  give  a  clue  to  the  parent  rock  or  rocks,  and  special 
features  in  the  minerals  may  also  afford  information.  Thus 
the  existence  of  fluid-,  glass-,  or  other  cavities  in  crystal- 
fragments,  the  presence  of  ru tile-needles  in  quartz-grains,  etc., 
may  tell  us  whether  the  minerals  in  question  originally  formed 
part  of  a  plutonic,  a  volcanic,  or  a  metainorphic  rock,  or  of 
several  different  rocks1.  Too  much  stress  must  not  be  laid  on 
the  rounding  of  grains  as  indicating  the  distance  of  their  source. 
Long-continued  drifting  to  and  fro  within  a  limited  area  may 
cause  more  attrition  than  many  thousand  miles  of  travel  in 
one  direction  :  further,  friction  is  much  more  effective  under 
subaerial  than  under  subaqueous  conditions.  Again,  sand- 
grains  must  often  be  furnished  ready-made  by  the  destruction 
of  older  arenaceous  deposits. 


FIG.  50.     '  TOP  GRIT  '  OR  UPPERMOST  BED  OF  THE  QUARTZITE  SERIES, 
NEAR  INCHNADAMFF,  SUTHERLAND  ;    x  20 : 

shewing  small  angular  quartz-grains  occupying  the  interstices  between 
the  larger  rounded  ones  [1665]. 

1  See,  e.g.,  Mackie's  investigation  of  the  Old  Red  Sandstone  of  Eastern 
Moray,  Tr.  Edin.  G.  S.  (1897)  vii,  148-172. 


CALCAREOUS  CEMENT  OF  SANDSTONES.      227 

The  coarseness  or  fineness  of  sandstones  may  vary  consider- 
ably. The  sifting  action  of  running  water  tends  to  collect  in 
one  place  grains  of  roughly  equal  dimensions,  but  some  sand- 
stones contain  grains  of  two  very  different  sizes,  the  smaller 
occupying  the  interspaces  between  the  larger  (fig.  50).  A  very 
common  size  for  the  grains  of  quartz  and  felspar  in  many 
sandstones  is  from  '01  to  '03  inch1. 

Authigenous  constituents.  In  addition  to  the  clastic 
grains,  sandstones  and  grits  contain  material  deposited  upon 
the  surfaces  of  the  grains,  or  filling  in  partially  or  wholly  the 
interstices  between  them,  and  thus  serving  to  bind  them  into 
a  coherent  rock.  Whether  formed  by  the  recrystallization  of 
calcareous  or  other  matter  laid  down  with  the  detritus,  by  the 
redeposition  of  material  dissolved  from  the  grains  themselves, 
or  by  the  introduction  in  solution  of  some  extraneous  substance, 
this  cement  must  be  regarded  as  formed  in  place,  and  its 
accumulation  constitutes  a  new  chapter  in  the  history  of  the 
rock.  The  cementing  medium  itself  is  usually  calcareous, 
ferruginous,  siliceous,  or  some  mixture  of  these. 

The  calcareous  cement  has  probably  been  in  most  cases 
deposited  in  the  form  of  mud,  comminuted  shells,  etc.,  with  the 
original  grains,  but  it  becomes  effective  as  a  binding  material 
only  after  some  amount  of  solution  and  redeposition,  which 
commonly  gives  it  a  more  or  less  evident  crystalline  texture. 
Exceptionally  a  crystalline  growth  of  calcite  may  enclose 
grains  in  ophitic  or  poecilitic  fashion,  as  in  the  Fontainebleau 
Sandstone  of  the  Paris  Miocene,  but  usually  the  calcareous 
cement  is  strictly  interstitial,  and  it  does  not  always  fully 
occupy  the  interspaces  between  the  grains.  In  rare  cases  other 
salts,  such  as  gypsum  and  barytes,  may  serve  as  a  cement. 

Many  sandstones  are  cemented  by  ferruginous  matter  or  a 
mixture  of  ferruginous  and  calcareous.  The  red  oxide  and  the 
brown  hydrated  oxide  of  iron  occur  in  this  way.  Frequently 
the  oxide  forms  a  thin  coating  or  pellicle  round  each  grain  of 
sand.  This  pellicle  can  be  removed  by  acid,  leaving  the  grains 
colourless. 

1  See  Bonney,  Rep.  Brit.  Ass.  for  1886,  p.  601,  and  Nature  (1886), 
xxxiv,  412. 

15—2 


228 


SILICEOUS   CEMENT   OF  SANDSTONES. 


The  clayey  material  (kaolin,  very  fine  mica,  etc.),  which 
occurs  interstitially  in  some  sandstones,  is  probably  to  a  great 
extent  authigenous,  representing  the  decomposition  of  felspar 
grains,  etc.  Similarly  a  chloritic  mineral  is  not  uncommon, 
and  may  be  derived  from  the  destruction  in  place  of  such 
minerals  as  hornblende  and  biotite. 

In  the  tougher  sandstones  and  grits  the  cementing  matter 
is  in  the  main  siliceous.  When  the  grains  are  angular  and  of 
various  sizes,  the  interspaces  may  be  very  small,  and  the  inter- 
stitial silica,  concealed  by  the  grains  and  perhaps  by  kaolin 
dust  or  iron-staining,  may  be  difficult  to  observe.  In  more  or 
less  porous  rocks,  the  little  cementing  matter  required  may  be 
provided  by  some  slight  solution  of  the  quartz-grains  them- 
selves at  the  points  where  they  press  on  one  another,  as 
suggested  by  Mr  Wethered  for  the  sandstones  of  the  Bristol 
coalfield. 

When  spaces  have  existed  between  the  original  grains,  it 
is  usually  seen  that  the  siliceous  cement  has  been  deposited  in 
crystalline  continuity  with  the  original  quartz  as  a  new  out- 
growth of  the  clastic  grains.  The  secondary  enlargement  of 


FIG.  51.     QUARTZ-GRAINS  FROM  PENRITH  SANDSTONE,  PENIUTH 
BEACON,  CUMBERLAND  ;    x  20  : 

shewing  a  secondary  outgrowth  of  quartz  with  crystal-faces  [1920]. 


SECONDARY   GROWTH   OF   SAND-GRAINS. 


229 


the  grains  is  verified  by  the  new  material  extinguishing  simul- 
taneously with  the  old  between  crossed  nicols.  Again,  many 
sandstones  which  have  not  been  compacted  into  hard  rocks 
exhibit  a  similar  new  growth  on  the  surfaces  of  the  grains ; 
and  in  this  case  (fig.  51 )  the  added  material  often  shews  good 
crystal  faces1  ('crystallized  sand').  The  enlargement  is  com- 
monly clearer  than  the  nucleus,  and  the  division  between  them 
is  marked  by  a  line  of  dusty  inclusions  or  by  a  thin  partial 
coating  of  some  deposit  older  than  the  outgrowth.  Though 
characteristic  of  quartz,  a  similar  outgrowth  is  occasionally 
found  on  fragments  of  felspar2  and  hornblende3. 


B 


FIG.  52.     QUARTZITE,  STIPERSTONES,  SHROPSHIRE  ;    x  50 : 

A  in  natural  light,  B  between  crossed  nicols.  The  grains  are  of 
rolled  quartz  with  an  occasional  turbid  felspar  (/),  and  the  interspaces 
are  filled  by  a  secondary  outgrowth  of  quartz  from  the  grains.  The 
shading  is  diagrammatic,  to  indicate  different  interference-tints.  A 
composite  grain  in  the  centre  shews  outgrowths  from  both  portions 
[224]. 

1  Sorby  (Address  cit.  supra,  62-64).    For  figures  see  B.  D.  Irving,  5th 
Ann.  Rep.  U.  S.  Geol.  Sur.  (1885)  pi.  xxx ;  Irving  and  Van  Hise,  Bull. 
No.  8  U.  S.  Geol.  Sur.  (1884)  pi.  n ;  Phillips,  Q.  J.  G.  S.  (1881)  xxxvii, 
pi.  ii. 

2  Irving,  I.  c.,  pp.  237-241,  and  44-47. 

3  Van  Hise,  A.  J.  S.  (1885)  xxx,  232-235. 


230  QUARTZITES. 

In  less  frequent  examples  new-formed  quartz  has  a  radial 
arrangement  about  original  grains,  or  is  oriented  independently. 
Again,  a  cement  of  cryptocrystalline  or  chalcedonic  silica  is 
known  in  some  rocks.  This,  however,  is  rather  characteristic 
of  volcanic  sandstones  and  conglomerates  in  regions  of  hot- 
spring  action  :  e.g.,  in  the  Yellowstone  Park  rolled  fragments 
of  obsidian  and  rhyolite  are  thus  cemented  into  a  hard  rock. 

When  a  deposit  originally  a  quartz-sand  becomes  completely 
compacted  by  an  interstitial  cement  of  secondary  quartz,  the 
result  is  a  quartzite  of  the  ordinary  type.  Such  rocks  often 
consist  wholly,  or  almost  wholly,  of  quartz,  but  in  a  thin  slice 
the  distinction  between  the  derived  grains  and  the  interstitial 
cement  comes  out  clearly.  Usually  the  new  quartz  is  a  cryst- 
alline outgrowth  from  the  grains,  the  space  between  two 
grains  being  occupied  by  quartz,  of  which  part  is  in  continuity 
with  one  grain,  part  with  the  other.  Between  crossed  nicols 
the  slice  therefore  assumes  the  appearance  of  an  irregular 
mosaic1  (fig.  52). 

Some  British  examples2.  The  forms  and  general 
characters  of  sand-grains  may  be  studied  in  modern  deposits3 
and.  in  the  sands,  not  yet  compacted  into  sandstone,  of  the 
later  geological  formations.  Among  the  materials  quartz,  as 
a  rule,  largely  predominates,  but  the  sands  of  our  modern 
coasts  are  locally  rich  in  other  minerals,  such  as  flint,  garnet, 
tourmaline,  magnetite,  ilmenite  (Cornwall),  silicified  wood 
(Eigg),  etc.  Most  sands  contain  a  small  proportion  of  certain 
heavy  minerals,  which  can  be  separated  by  special  methods. 
In  the  fine-grained  Bagshot  Sands  of  Hampstead  Heath  and 


1  For  coloured  figures  see  Teall,  pi.  XLV,  fig.  2 ;  XLVI,  fig.  1 ;  Irving 
(cit.  supra),  pi.  xxxi ;  Irving  and  Van  Hise,  On  Secondary  Enlargements 
of  Mineral  Fragments  (1884),  Bull.  No.  8  U.  S.  Geol.  Sur.  pi.  m-vi. 

2  Interesting    information   concerning   British  arenaceous   rocks    is 
contained  in   Sorby's   Presidential  Address,  quoted  above,  and  earlier 
papers  (Proc.  Yorks.  Geol.  and  Pol.  Soc.,  etc.).     See  also  J.  A.  Phillips, 
Q.  J.  G.  S.  (1881)  xxxvii,  6-27;  Bonney,  Nature  (1886),  xxxiv,  442-451, 
and  Rep.  Brit.  Ass.  for  1886,  601-621. 

3  For  an  account  of  the  sands  and  other  deposits  now  forming  in  the 
Irish  Sea  see  Herdman,  Rep.  Brit.  Ass.  for  1894,  328-339,  and  Pr.  Liverp. 
G.  S.  (1894)  vii,  171-182  ;  Herdman  and  Lomas,  ibid.  (1898)  viii,  205-232. 


BRITISH    GLACIAL   AND   RECENT   SANDS.  231 

of  High  Beech  in  Essex  Mr  Dick1  found  up  to  4  per  cent,  of 
dense  minerals,  including  magnetic  iron  ore,  zircon,  rutile,  and 
tourmaline.  Many  sands  contain  small  quantities  of  these 
and  other  special  minerals  (garnet,  cyanite,  anatase,  etc.). 
The  basal  bed  of  the  Thanet  Sands  contains  20  per  cent,  of 
flint  in  sharply  angular  chips,  with  quartz,  glauconite,  and 
numerous  other  minerals2.  The  flint  is  of  course  derived  from 
the  Chalk. 

The  form  of  quartz-grains  depends  in  great  measure  upon 
their  source,  whether  directly  from  crystalline  rocks  or  from 
older  sandstones  or  grits.  Thus  the  Glacial  sands  of  the  York- 
shire coast,  which  must  come  chiefly  from  crystalline  rocks, 
have  sharply  angular  shapes,  and  the  grains  on  the  modem 
beaches  of  that  coast,  most  of  which  are  doubtless  washed  out 
of  the  Glacial  accumulations,  are  scarcely  more  abraded.  On 
the  other  hand,  modern  sands  on  the  south-east  coast  of 
England,  derived  very  largely  from  older  arenaceous  deposits, 
have  a  considerable  proportion  of  rounded  grains.  On  the 
north-west  coast  both  Glacial  and  modern  sands  often  contain 
extremely  rounded  grains,  explained  as  being  derived  from 
the  '  millet-seed '  sandstones  of  the  Trias,  but  these  are  mixed 
with  angular  quartz  in  various  proportions.  The  grains  of  the 
sand-dunes  on  pur  coasts  are  much  less  rounded  than  those  of 
desert  sands. 

The  Mesozoic  formations  afford  numerous  examples  of 
calcareous  and  ferruginous  cements.  Thus  the  Calcareous 
Grits  of  Yorkshire  have  a  cement  of  calcite,  often  stained  or 
mixed  with  iron-oxide,  and  some  of  them  might  with  equal 
propriety  be  named  impure  gritty  limestones.  The  Kellaways 
Rock  has  usually  a  ferruginous  cement  (fig.  53,  B).  In 
the  Lower  Greensand  of  the  eastern  counties  the  cement  is 
sometimes  largely  ferruginous,  with  a  little  interstitial  quartz 
(*  carstone '  of  Hunstanton),  but  in  many  cases  is  of  granular 
calcite,  which  may  be  iron- stained.  Occasionally  the  calcite 
builds  large  plates  enclosing  many  of  the  partly  rolled  quartz- 
grains  (Spilsby  in  Lincolnshire,  Copt  Point  near  Folkestone). 

1  Nature  (1887),  xxxvi,  91,  92  ;  Teall,  pi.  XLIV  ;  cf.  Fouqu6  and  L^vy, 
'  2  Miss  Gardiner,  Q.  J.  G.  S.  (1888)  xliv,  755-760. 


232  BRITISH   CRETACEOUS   SANDSTONES. 

Many  of  these  rocks  have  little  grains  of  bright  green 
glauconite  with  various  rounded  shapes,  explained  as  casts  of 
foraminifera.  Another  feature  is  the  occurrence  of  little  round 
oolitic  grains  of  dark  brown  iron-ore  ('carstone'  of  Hunstanton, 
and  Roslyn  Hill,  Ely).  These  grains  have  a  concentric-shell 
structure,  and,  when  dissolved  in  acid,  leave  a  siliceous  skeleton 
(fig.  53,  A).  Zircon  crystals  are  among  the  denser  consti- 
tuents1. The  best  British  examples  of  glauconitic  sands  come 
from  the  base  of  the  Cretaceous  in  Antrim.  The  glauconite 
grains,  unusually  large  and  abundant,  are  all  casts  of  fora- 
miniferal  chambers2.  The  glauconitic  sands  of  the  Upper 


B 


V 

FIG.  53;    x  20. 

A.  Calcareous  grit  in  Lower  Greensand,  Roslyn  Hill,  Ely  :  shewing 
subangular  quartz,  with  a  few  glauconite  casts  of  foraminifera  (gl),  and 
derived  oolitic  grains  of  dark  brown  iron -ore  (•/),  cemented  by  a  matrix  of 
granular  calcite  [1799].  B.  Ferruginous  grit,  Kellaways  Rock,  South 
Cave,  Yorkshire  :  shewing  angular  quartz-grains  in  a  cement  mainly  of 
iron-oxide  [1797]. 

Greensand  in  Wiltshire,  Dorset,  Devon,  and  the  Isle  of  Wight 
are  chiefly  of  coarse  quartz-sand  with  fragments  of  felspar  and 

1  Hume,  Q.  J.  G.  S.  (1894)  1,  679  (Bargate). 

2  Ibid.  (1897)  liii,  569-571. 


BRITISH    RED   SANDSTONES.  233 

mica,  but  large  glauconite  grains   are   abundant.     There  is 
often  a  calcareous  cement1. 

The  Upper  Paleozoic  grits  and  sandstones  of  this  country 
often  have  a  cement  largely  ferruginous  or  consisting  of  iron- 
oxide  and  quartz.  In  the  Devonian  of  South  Devon  are  fine- 
grained sandstones  which,  with  predominant  quartz,  have  little 
flakes  of  mica,  some  felspar,  and  small  granules  of  tourmaline, 
indicating  the  source  of  the  material :  the  interstitial  matter  is 
for  the  most  part  ferruginous.  Much  of  the  Old  Bed  Sand- 
stone shews  the  investing  pellicle  of  ferric  oxide  around  each 
grain. 

This  latter  feature  and  numerous  other  points  of  interest 
may  be  studied  in  many  parts  of  the  New  Red  Sandstones. 
In  particular,  quartz-grains  with  a  secondary  outgrowth  having 
crystal-faces  are  common  at  various  horizons  of  the  Keuper 
and  Bunter2  of  Shropshire,  Cheshire,  etc.,  and  are  also  excep- 
tionally well  exhibited  in  some  coarse-grained  beds  of  the 
Penrith  Sandstone  (Penrith  Beacon,  Cumberland),  (fig.  51). 
In  some  cases  a  pellicle  of  iron-oxide  coats  the  new  crystal- 
growth,  and  must  then  be  long  posterior  to  the  date  of  the 
strata.  Red  sandstones  are  often  of  quite  yielding  consistency, 
even  when  the  interstices  are  occupied  by  quartz.  This  is 
because  of  the  coating  of  iron-oxide  intervening  between  the 
interstitial  quartz  and  the  original  grain.  By  treatment  with 
acid,  the  irregularly  shaped  patches  of  interstitial  quartz  were 
isolated  by  Mr  Phillips  from  the  'millet-seed'  sandstones  of  the 
Trias.  In  these  beds  the  perfectly  rounded  form  of  the  original 
grains  is  attributed  to  their  having  been  true  desert-sands3. 

Many  Carboniferous  grits  have  sharply  angular  grains,  and 
were  probably  derived  directly  from  crystalline  rocks.  The 

1  On  these  and  other  arenaceous  rocks  of  the  Upper  Cretaceous  see 
W.  Hill  in  Mem.  Geol.  Sur.,  Cret.  Rocks  Brit.,  vol.  i.  (1900)  chap.  xxv. 

2  For  descriptions  of  Triassic  sandstones  from  the    Vale   of  Clwyd, 
Cheshire,  and  Lancashire,  see  Morton,  Geology  of  Liverpool  (2nd  ed.  1891) 
129-132  ;  M.  Reade,  Pr.  Liv.  G.  S.   (1892)  vi,   374-386  ;   Dickson  and 
Holland,  ibid.  (1896)  vii,  449-451 ;   Moore,  ibid.   (1898)   viii,  241-265 ; 
Lomas,  ibid.  265-267,  pi.  xm. 

3  Cf.  Mackie  on  the  Reptiliferous  Sandstone  of  Elgin,  Tr.  Edin.  G.  S. 
(1897)  vii,  166,  pi.  xix,  fig.  2. 


234  BRITISH    PALEOZOIC   GRITS. 

coarse-grained  Millstone  Grit  of  south  Yorkshire1  has  highly 
irregular  quartz-grains  poor  in  fluid-cavities.  There  is  not 
much  fresh  felspar,  but  argillaceous  matter  between  the  quartz  • 
grains  seems  to  represent  it.  The  hard  '  ganister '  has  angular 
quartz-grains  which  fit  so  closely  together  as  to  obscure  the 
small  amount  of  siliceous  cement,  and  the  same  is  true  of  the 
grits  of  the  Bristol  coal-field.  In  some  beds  in  the  Coal- 
Measures  numerous  flakes  of  muscovite  lying  parallel  to  the 
lamination  impart  a  fissile  character  to  the  rocks  (Bradford 
Flags,  etc.).  The  spaces  between  the  grains  are  often  obscured 
by  kaolin.  Kaolin  and  relics  of  reddish  orthoclase,  with  a 
little  mica  and  sometimes  tourmaline,  are  found  in  the  Millstone 
Grit  of  south-west  Lancashire2,  which  consists  mainly  of 
angular  quartz-grains  of  very  variable  size  ('2  to  "005  inch) 
with  crystalline  outgrowths  not  very  common.  In  the  Cefn-y- 
Fedw  Sandstone  of  Denbighshire  and  Flintshire3  the  grains 
are  angular  to  rounded,  and  more  often  have  secondary  out- 
growths with  crystal-faces. 

The  Lower  Palaeozoic  and  older  arenaceous  rocks  are,  as  a 
rule,  thoroughly  compacted,  the  cement  being  for  the  most  part 
siliceous.  Mr  Phillips  found  the  quartz-cement  of  various 
Cambrian  and  Silurian  grits  (Barmouth,  Harlech4,  Aberystwith, 
Denbighshire)  permeated  by  a  moss -like  growth  of  a  green 
chloritic  mineral.  Both  coarse  and  fine-textured  rocks  are 
included.  The  quartz-grains  are  angular  or  partly  rounded, 
and  frequently  contain  needles  of  rutile  and  tourmaline :  fluid- 
pores  are  present  in  some,  absent  in  others.  Some  of  the  grits 
have  plenty  of  felspars,  while  pyrites,  garnet,  and  micas  are 
occasionally  noted.  Specimens  of  the  grits  of  Skiddaw  and  of 
the  Isle  of  Man  (Santon)  shew  fragments  of  slate  and  lava 
among  the  partly  rolled  quartz  and  turbid  felspars.  The 
Ingleton  rock  in  Yorkshire  is  a  grit  containing  volcanic  material 
as  well  as  grains  and  pebbles  of  quartz,  felspars,  and  various 

1  Sorby,  Pr.  Yorks.  Geol.  Pol.  Soc.  (1859)  iii,  669-675.     On  the  Mill- 
stone Grit  of  the  Forest  of  Dean  see  Wethered,  Pr.  Cottesw.  F.  N.  Club 
(1883),  viii,  25-27,  with  plates. 

2  Morton,  Proc.  Liverp.  G.  S.  (1887)  v,  280-283. 

3  Ibid.  271-279. 

4  Cf.  Greenly,  Tr.  Edin.  G.  S.  (1897)  vii,  254-258. 


BRITISH   PALEOZOIC   GRITS. 


235 


lavas1.  Volcanic  grits  of  finer  texture  occur  in  the  upper  part 
of  the  Ordovician  near  Shap  Wells,  Westmorland,  and  these 
contain  also  calcareous  matter. 

The  older  sandstones  of  the  Bangor  and  Caernarvon  district 
and  of  parts  of  Anglesey  are  rather  coarse-grained,  consisting 
of  well-rounded  to  subangular  quartz  with  plenty  of  felspar. 
The  latter  mineral  is  often  decomposed,  and  its  clayey  de- 
composition-products wedged  in  between  the  quartz-grains, 
obscuring  the  siliceous  cement  (fig.  54).  Some  of  the  rocks, 
however,  have  comparatively  fresh  felspar :  a  Silurian  grit  at 
Drys-lwyn-isaf,  south  of  Parys  Mountain,  consists  almost 
wholly  of  grains  of  oligoclase  closely  packed  together.  The 
prevalent  type  of  the  Torridon  Sandstone  is  an  example  of  a 
coarse  sandstone  rich  in  felspar.  Besides  rolled  quartz-grains 
often  composite,  it  has  others  of  microcline  and  fragments  of 
quartzite  and  pegmatite. 


FIG.  54.     SANDSTONE,  FACHELL,  NEAR  LLANDEINIOLEN,  CAERNARVON- 
SHIRE ;   x  20. 

Besides  the  well-rolled  quartz-grains  with  many  rows  of  fluid-pores, 
felspar  is  represented  (/).  This  is  largely  decomposed,  the  resulting 
clayey  material  being  squeezed  between  the  quartz -grains  [282]. 

1  Tate,  Rep.  Brit.  Ass.  for  1890,  800. 


236  BRITISH   QUARTZITES. 

The  best  examples  of  quartzites  in  England  are  those  of 
Hartshill  in  Warwickshire  and  the  Lickey  Hills  in  Worcester- 
shire, probably  of  pre-Cambrian  age1,  and  the  Stiperstones  in 
Shropshire  (Ordovician) 2.  All  these  consist  essentially  of  rolled 
quartz-grains,  usually  about  '02  to  '03  inch  in  diameter,  with 
only  very  subordinate  felspar,  united  by  a  clear  quartz-cement 
which  is  of  the  nature  of  a  crystalline  outgrowth  from  the 
grains  (fig.  52).  A  series  of  quartzites  forms  the  lower  part  of 
the  Cambrian  in  the  Assynt  district,  Sutherland.  Some  beds 
contain  pebbles,  and  are  indeed  cemented  conglomerates.  The 
uppermost  bed  ('Top  Grit')  shews  large  well-rolled  quartz- 
grains,  about  *05  inch  in  diameter,  with  smaller  subangular 
grains  between  them.  The  remaining  space,  occupied  by  the 
siliceous  cement,  is  obscured  by  opaque  dust  (fig.  50).  Good 
quartzites,  probably  of  Cambrian  age,  occur  at  Bray  Head  and 
Howth  near  Dublin3.  In  some  districts  quartzites  have  been 
formed  at  higher  geological  horizons.  Thus,  the  *  Moor  Grit,' 
a  conspicuous  coarse-grained  bed  in  the  Lower  Oolites  of  the 
Yorkshire  moors,  is  rather  a  quartzite  than  a  grit ;  and  the 
same  is  often  true  of  the  well-known  'sarsen  stones'  or  'grey- 
wethers  '  of  Wiltshire,  Dorset,  etc. 4 

Of  American  arenaceous  rocks  a  number  of  typical  examples 
are  described  by  Diller5.  Interesting  secondary  outgrowths  of 
clastic  grains,  often  with  good  crystal-facets,  are  seen  in  the 
Potsdam  Sandstone6  of  Michigan  and  Wisconsin  and  in  the 
Huronian  quartzites7  of  the  same  region.  Conglomerates  from 
Minnesota  shew  similar  outgrowths  of  hornblende  fragments8. 

1  Teall,  pi.  XLV,  fig.  2,  XLVI,  fig.  1,  and  Pr.  Phil.  Soc.  Birm.  (1882)  iii, 
194-202;  Watts,  Summary  of  Progress  Geol.  Sur.  for  1897,  68,  and  Pr. 
Geol.  Ass.  (1898)  xv,  393,  397. 

2  Rutley,  Pr.  Liverp.  G.  S.  (1885)  v,  381. 

3  Sollas,  Sci.  Pr.  Roy.  Dubl.  Soc.  (1892)  vii,  174-184,  pi.  xv  ;  Pr. 
Geol.  Ass.  (1893)  xiii,  91-93,  pi.  in. 

4  Cf.  Judd,  G.  M.  1901,  1,  2. 

5  Diller,  Educ.  Series  Rock- Specimens,  59-64,  74-84,  pi.  ix,  x,  xni. 

6  Irving,  A.  J.  S.  (1883)  xxv,  401-411  ;  5th  Ann.  Rep.   U.  S.  Geol. 
Sur.  (1885)  pi.  xxx  ;  Diller,  79,  80,  pi.  x.     A  number  of  figures  of  Pots- 
dam Sandstones  are  given  by  Buckley,  Build,  and  Ornam.  Stones,  Wis.t 
Bull.  4  of  Wis.  Geol.  and  Nat.  Hist.  Sur.  (1898)  pi.  LXIV-LXVII. 

7  5th  Ann.  Rep.  U.  S.  Geol.  Sur.  (1885)  pi.  xxxi,  and  Bull.  No.  8  U.  S. 
Geol.  Sur.  (1884)  pi.  m-vi. 

8  Van  Hise,  A.  J.  S.  (1885)  xxx,  231-235. 


CHAPTER  XVII. 
ARGILLACEOUS  ROCKS. 

THE  name  clay  is  used  for  argillaceous  deposits  which  still 
retain  enough  moisture  to  be  plastic.  By  the  loss  of  most  of 
their  uncoinbined  water  and  by  other  more  important  changes 
these  pass  into  mudstones,  shales,  and  slates.  Of  these  terms, 
mudstone  is  correctly  used  when  the  rock  has  no  marked  fissile 
character,  shale  when  it  splits  along  the  original  lamina?  of 
deposition,  and  slate  when  the  original  lamination  has  been 
superseded  as  a  direction  of  weak  cohesion  by  a  new  structure 
(slaty  cleavage,  Fr.  schistositd,  Ger.  Trans  versalschieferung). 
The  Continental  geologists  do  not,  as  a  rule,  observe  this 
distinction,  but  include  shales  and  slates  under  the  same  name 
(Fr.  schiste,  Ger.  Schiefer,  Norw.  skiffer). 

Among  slates  it  has  been  usual  to  distinguish  ckty-slates 
(Thonschiefer,  lerskiffer),  in  which  the  material  was  supposed 
to  be  largely  detrital  matter  without  important  new  formation 
of  minerals,  and  phyllites  (Tr.  phyllade),  in  which  the  rocks  are 
largely  or  totally  reconstituted  in  place  (aided,  at  least,  by 
pressure).  It  is  now  becoming  evident,  however,  that  in  clay- 
slates,  and  even  in  clays  and  shales,  there  has  often  been  a 
considerable  amount  of  mineral  change  in  place ;  so  that  no 
v&ry  sharp  line  can  be  drawn  between  clay- slates  and  phyllites. 
The  typical  glossy  phyllites  are  essentially  mica-schists  on  a 
small  scale,  and  may  be  described  as  micro-crystalline  schists. 
We  shall  find  it  convenient  to  include  them  here,  although  we 
thereby  anticipate  their  place  under  the  head  of  dynamic 
metamorphism. 


238  CONSTITUENTS   OF   CLAYS,   ETC. 

Constituent  minerals.  Owing  to  the  extremely  small 
dimensions  of  the  elements,  it  is  usually  a  matter  of  great 
difficulty  to  identify  with  certainty  all  the  constituents  of 
clays,  shales,  or  slates.  Speaking  generally,  these  constituents 
include  some  of  derived  or  detrital  origin  (allothigenous), 
which  were  either  primary  minerals  or  decomposition-products 
in  the  parent  rock-masses,  and  others  of  secondary  origin, 
formed  in  place  (authigenous).  As  regards  the  latter,  doubt 
may  exist  in  particular  cases  as  to  how  far  the  secondary 
recombinations  have  been  induced  by  pressure  (dynamic  meta- 
morphism).  In  many  fine-grained  slates  no  constituents  are 
seen  which  can  be  set  down  with  confidence  as  purely  detrital. 
In  all  cases  very  thin  sections  and  high  magnifying  powers 
must  be  used.  Some  of  the  denser  accessory  minerals  may 
be  isolated  from  powder  by  heavy  solutions,  or  merely  by 
washing1. 

The  detrital  elements  may  include  granules  of  quartz,  and 
less  frequently  of  felspars,  and  scales  of  mica,  with  minute 
crystals  of  such  accessories  as  zircon.  The  little  flakes  of 
biotite  shew  more  or  less  decomposition  :  Mr  Hutchings  finds 
that  they  give  rise,  not  to  chlorite,  but  to  epidote  in  minute 
superposed  tablets  of  light  yellow  colour.  The  iron -oxides 
separate  out  as  limonite.  Carbonates  may  occur  in  varying 
proportion.  Many  argillaceous  rocks  contain  a  considerable 
quantity  of  carbonaceous  matter,  finely  granular  and  for  the 
most  part  opaque  :  slices  may  be  bleached  by  incineration 
on  platinum  foil.  The  pyrites  which  occurs  in  many  slates, 
sometimes  in  relatively  large  crystals,  is  of  secondary  origin, 
and  is  perhaps  due  to  the  reduction  of  iron-compounds  in  the 
presence  of  organic  matter.  The  glauconite  of  some  argillaceous 
deposits  has  also  been  formed  in  place2. 

The  ordinary  fine-grained  argillaceous  rocks  consist  in 
considerable  part  of  an  exceedingly  fine-textured  base  or 
paste,  very  difficult  to  resolve,  in  which  any  truly  detrital 

1  Cf.  Teall,  M.  M.  (1887)  vii,  201-204.    For  a  method  of  studying  fine 
incoherent  sediments,  see  Hutchings  on  Sediments  dredged  from  the 
English  Lakes,  G.  M.  1894,  300-303. 

2  See  W.  Hill  on  the  micro-structure  and  mineral  ingredients  of  the 
Gault,  Mem.  Geol.  Sur.,  Cret.  Rocks  Brit.,  vol.  i  (1900),  chap.  xxiv. 


FINE   PASTE   OF   CLAYS,   ETC.  239 

elements  or  their  evident  alteration-products  are  embedded. 
The  nature  of  this  paste  has  not  yet  been  made  out  in  any 
large  number  of  cases.  It  was  formerly  regarded  as  consisting 
essentially  of  hydrated  silicate  of  alumina  (kaolin),  etc.  Care- 
ful studies  of  various  clays,  shales,  and  slates  lead,  however, 
to  the  conclusion  that  the  material  is  to  a  great  extent  a 
very  finely  divided  micaceous  substance  of  secondary  origin  ; 
and  this  is  confirmed  by  chemical  analyses  of  the  rocks, 
which  often  shew  a  considerable  content  of  alkalies.  Ac- 
cording to  Mr  Hatchings1,  this  main  constituent  of  the 
fine-grained  base  is  in  ordinary  clays  and  shales  an  impure, 
pale,  greenish-yellow  mica ;  while  in  slates,  where  crystalline 
reconstruction  is  more  advanced,  it  has  given  place  to  a 
mixture  of  pure  muscovite  and  a  chlorite-mineral,  the  two 
often  in  very  intimate  association.  In  rocks  not  completely 
regenerated  there  may  be  observed  in  addition  much  indeter- 
minable finely  granular  matter,  which  may  be  conjectured  to 
represent  the  finest  powder  of  quartz,  felspar,  etc.,  and  perhaps 
kaolin  or  other  products.  A  highly  characteristic  feature  of 
the  paste  is  the  presence  of  an  enormous  number  of  minute 
needles  of  rutile  (' clay-slate-needles')2.  On  account  of  their 
very  small  breadth  and  very  high  refractive  index,  the  needles 
often  appear  as  opaque  lines,  but  the  larger  ones  may  be 
transparent.  The  rutile  is  generally  regarded  as  of  secondary 
origin,  being  produced  in  place  in  association  with  the  mica, 
etc.,  the  titanic  acid  being  furnished  by  derived  biotite.  Since 
the  changes  which  gave  rise  to  these  secondary  products  have 
operated  in  clays  as  well  as  in  slates,  they  cannot  be  held  to 
imply  any  advanced  dynamic  metamorphism,  but  they  may 
still  be  favoured  by  pressure. 

Many  slates  seem  to  shew  by  their  chemical  composition 
the  presence  of  secondary  free  silica  (in  addition  to  any  evident 
detrital  quartz  which  they  may  contain).  This  is  sometimes 
seen  as  a  quartz-cement,  tending  to  form  little  veins  and 

1  G.  M.  1896,  312,  313.     This  author  points  out  the  advantages  of 
cutting  slices  from  a  specimen    previously  ignited    to    redness.     The 
resulting  dehydration   causes  the  chloritic  substance  to  become  more 
opaque,  or  assume  a  deeper  colour,  while  impure  mica  is  less  affected, 
and  the  pure  muscovite  unchanged. 

2  Cf.  Teall,  M.  M.  (1887)  vii,  201-204 ;  Cohen  (3),  pi.  in,  fig.  4. 


240  SLATY   CLEAVAGE. 

patches ;  in  other  cases  opal  has  been  supposed  to  occur,  and 
indeed  amorphous  silica  may  be  dissolved  out  by  caustic 
potash. 

In  some  rocks,  especially  the  Glacial  tills,  we  must  suppose 
that  a  large  part  of  even  the  most  impalpable  material  is  of 
detrital  origin.  Thus  in  the  tills  of  the  Boston  basin,  Massa- 
chussetts,  Crosby1  found  that  about  four-fifths  of  the  finest 
grade  of  material  was  not  what  is  commonly  understood  by 
clay,  but  what  he  terms  'rock-flour/  i.e.  the  most  minute 
particles  of  pulverised  quartz  and  other  rock-forming  minerals, 
not  chemically  decomposed. 

Structures.  Argillaceous  rocks  in  general  have  a  paral- 
lel arrangement  of  their  constituent  elements  which  is  usually 
sufficiently  marked  to  impart  a  fissile  character  to  the  mass. 
Slices  parallel  and  perpendicular  to  the  direction  of  fissile 
structure  should  be  compared.  In  shales  a  large  proportion 
of  the  minute  constituent  elements  lie  with  their  flat  faces  or 
long  axes  parallel  to  the  layers  of  deposition.  In  true  slates, 
i.e.  rocks  with  a  superinduced  cleavage-structure,  they  have 
taken  up  a  new  direction  along  planes  (cleavage-planes)  per- 
pendicular to  the  maximum  compression  by  which  the  rock 
has  been  affected. 

The  effect  of  this  compression,  accompanied  by  a  certain 
partially  compensating  expansion  along  the  cleavage-planes,  is 
well  seen  in  the  deformation  of  concretionary  spots  of  colour, 
etc.  A  spherical  spot  becomes  distorted  into  an  ellipsoid.  A 
hard  unyielding  body,  such  as  a  crystal  of  pyrites  or  magnetite 
embedded  in  the  rock,  gives  rise  to  curious  phenomena.  The 
matrix  flows  past  the  crystal,  leaving  a  roughly  eye-shaped 
space'2.  Such  crystals  have  in  many  cases  been  originally 
coated  with  an  envelope  of  chlorite,  which  adheres  to  the 
matrix  and  is  torn  away  from  the  crystal.  The  intervening 
space  is  subsequently  filled  by  infiltration  with  crystalline 
quartz  (fig.  55,  A). 

Various  structures,  of  frequent  though  local  occurrence  in 

1  Proc.  Bost.  Nat.  Hist.  Soc.  (1890)  xxv,  115-172. 

2  G.  M.  1889,  396,  397. 


FALSE   CLEAVAGE-STRUCTURES.  241 

fine-grained  beds,  may  be  styled  '  false'  and  incipient  cleavages\ 
They  consist  sometimes  in  a  parallel  system  of  microscopic 
faults,  sometimes  in  a  regular  system  of  minute  folds.  These 


B 


FIG.   55. 

A.  Slate  with  crystal  of  pyrites,  Penrhyn,  near  Bangor  ;  x  5.  The 
crystal  is  surrounded  by  an  'eye'  of  chlorite  and  quartz,  as  described. 
The  mass  of  the  slate  contains  little  light  spots,  which  have  been 
deformed  into  an  elliptic  shape  [501].  B.  False  cleavage  in  Skiddaw 
Slate,  Brownber,  near  Appleby ;  x  20.  The  system  of  minute  parallel 
folds  causes  a  direction  of  weakness  almost  equivalent  to  cleavage  [913]. 

often  give  a  tendency  to  the  rock  to  split  along  definite  planes, 
viz.  the  fault-surfaces  or  the  limbs  of  the  folds  (fig.  55,  J?). 
Dr  Sorby2  has  shewn  that  such  structures  may  be  a  step 
towards  a  true  slaty  cleavage.  They  may  also,  however, 
occur  as  later  structures  crossing  a  true  cleavage  (e.g.  in 
various  Ardennais  slates  and  phyllites),  and  they  are  common 
in  some  fine-textured  mica-schists.  They  are  often  interesting 
as  reproducing  on  a  minute  scale  the  characteristic  structures 
of  mountain-ranges,  such  as  the  gradual  passage  of  an  over- 
fold  into  an  overthrust  fault,  the  relation  of  faults  to  anticlines, 

1  Eep.  Brit.  Ass.  for  1885,  836-841.  Some  writers  have  used  the 
terms  'close-joints  cleavage '  (Sorby), '  Ausweichungsclivage  '  (Heim),  and 
'  strain-slip-cleavage  '  (Bonney)  for  structures  of  this  kind. 

a  Q.  J.  G.  S.  (1880)  xxxvi,  Proc.  72,  73. 

ii.  P.  16 


242  DEEP-SEA   'RED  CLAY.' 

etc.  A  frequent  result  of  shearing  movement  in  finely  laminated 
rocks  is  the  formation  of  minute  oblique  folds  inclined  at  about 
45°  to  the  lamination  :  these  are  pushed  over  until  at  about  30° 
they  pass  into  little  faults,  and  the  faults  may  be  further  pushed 
over  until  they  are  lost  in  a  general  parallel-structure. 

Illustrative  examples.  Before  describing  some  of  the 
commoner  types  of  argillaceous  rocks,  we  may  mention  one  of 
which  very  little  is  known  among  consolidated  strata.  It  is 
represented  among  deposits  now  forming  by  the  abyssal  red 
clay  which  covers  large  areas  of  the  ocean-floor  below  a  depth 
of  2200  fathoms.  This  deep-sea  clay  is  derived  mainly  from 
the  destruction  of  volcanic  products  by  the  chemical  action  of 
sea-water.  Minute  fragments  of  volcanic  rocks  and  minerals 
are  mixed  with  decomposition-products  and  with  a  few  siliceous 
organisms  (radiolarians,  etc.).  The  brownish-red  colour  is  due 
to  disseminated  limonite.  Minute  crystals  of  the  lime-zeolite 
phillipsite  or  christianite  are  common1,  and  manganese-nodules 
of  various  sizes  occur.  There  may  also  be  a  few  corroded 
tests  of  foraminifera.  Messrs  Harrison  and  Jukes-Browne2 
found  that  about  two-thirds  of  a  typical  '  red  clay '  consists 
of  fine  argillaceous  matter  derived  from  the  destruction  of 
basic  pumice  or  palagonite.  The  rest  is  chiefly  disintegrated 
(but  not  decomposed)  acid  pumice ;  while  5  per  cent,  of  the 
clay  is  matter  of  organic  origin,  principally  colloid  silica. 
The  red  and  yellow  deep-sea  clays  of  the  Tertiary  in  the 
Barbados  have  a  very  similar  constitution3.  Other  rocks 
comparable  with  the  abyssal  red  clay  have  been  described 
from  the  Solomon  Islands4  and  from  Trinidad5. 

These  deep-sea  argillaceous  deposits  hare  characters  which 
distinguish  them  from  those  derived  from  the  waste  of  land- 
areas.  The  particles  are  of  excessive  minuteness  and  markedly 
angular  in  shape6.  The  minerals  recognizable  are  those  most 

1  See  Murray  and  Benard,  '  Challenger '  Report,  Deep- Sea  Deposits 
(1891),  pi.  xxn. 

2  Q.  J.  G.  S.  (1895)  li,  314-321. 

3  Cf.  Miss  Raisin,  Q.  J.  G.  S.  (1892)  xlviii,  180-182. 

4  Guppy,  The  Solomon  Is.,  their  Geology,  etc.  (1887)  81,  82. 

5  Gregory,  Q.  J.  G.  S.  (1892)  xlviii,  539. 

6  Murray  and  Renard,  I.  c.,  pi.  xxvi,  xxvu,  figs.  1-4  ;  contrast  with 


CHINA-CLAY:    LATERITE.  243 

common  as  constituents  of  volcanic  rocks,  such  as  felspar  and 
augite,  rarely  quartz ;  while  such  minerals  as  zircon,  tourmaline, 
etc.,  are  absent.  Usually  a  very  large  proportion  of  the 
material  consists  of  angular  chips  of  volcanic  glass  and 
elongated  fragments  derived  from  the  breaking  up  of  pumice 
with  capillary  pores. 

As  another  somewhat  peculiar  type  of  clay  may  be  men- 
tioned the  china-clay  of  Cornwall,  which  seems  to  consist 
essentially  of  the  mineral  kaolin1.  This,  in  its  most  recogniz- 
able form2,  builds  minute  colourless  scales,  sometimes  with 
hexagonal  outline,  and  of  such  refractive  index  and  birefring- 
ence as  closely  to  resemble  mica.  It  appears,  however,  from 
Mr  Collins's  account3  that  these  distinct  flakes  do  not  form 
any  large  part  of  the  finely  divided  material  in  the  typical 
occurrences  in  Cornwall.  Besides  quartz,  mica,  and  other 
impurities,  tourmaline  is  found  in  some  rocks  composed  largely 
of  kaolin,  and  its  production  was  perhaps  connected  with  the 
process  of  '  kaolinization '  of  felspathic  rocks4.  In  addition  to 
the  proper  china-clays,  formed  more  or  less  in  situ,  there  are 
derived  clays  of  similar  composition,  such  as  those  of  Bovey 
Tracey. 

Under  certain  conditions,  not  yet  made  clear,  it  appears 
that  decaying  igneous  rocks  may  be  deprived  more  or  less 
completely  of  their  combined  silica,  as  well  as  the  alkalies  and 
dioxides,  the  alumina  remaining  in  the  form  of  hydrate,  often 
with  ferric  hydrate.  The  bauxite-clays  of  Antrim  are  of  this 
type,  and  probably  result  from  the  subaerial  decomposition  of 
basalt  almost  in  place.  Where  quartz-bearing  rocks  have  been 
subjected  to  this  kind  of  change,  quartz-sand  remains  mixed 
with  the  aluminium  and  iron  hydrates.  Much  of  the  so-called 
later ite  of  India  and  other  tropical  countries  seems  to  be  of 
this  nature. 

1  Some  writers  apply  the  name  kaolin   to   the  clay  itself,  and  use 
'  kaolinite '  for  the  mineral.     Collins  uses  the  name  4  carclazite  '  for  the 
true  kaolin-clay  and  'petuntzite'  for  a  less  altered  variety  still  retaining 
relics  of  undestroyed  felspar. 

2  See  Dick,  M.  M.  (1888)  viii,  15-27,  pi.  in. 

3  M.  M.  (1887)  vii,  205-214  ;  Teall,  pi.  XLIV,  fig.  5. 

4  Butler,  M.  M.  (1887)  vii,  79,  80 ;  etc. 

16—2 


244  FIRE-CLAYS:    WELSH   SLATES. 

We  pass  on  to  the  consideration  of  clays  and  slates  of 
more  ordinary  constitution,  selecting  only  a  few  examples 
which  may  be  regarded  as  typical1. 

A  minute  study  of  typical  argillaceous  rocks  has  been 
made  by  Mr  Hutchings  in  the  case  of  the  fire-clays  of  the 
Newcastle  Coal-measures2.  The  rocks  are  laminated,  and  in- 
clude coarser  and  finer  beds.  The  material  of  true  detrital 
origin  is  most  abundant  in  the  coarser  beds.  It  seems  to  be 
derived  from  the  destruction  of  granite,  and  consists  of  granules 
of  quartz  averaging  *002  to  '003  inch  in  diameter,  granules  of 
felspar,  biotite  flakes  from  '01  inch  downward,  with  the 
epidotic  alteration,  less  abundant  muscovite,  and  accessory 
zircon,  etc.  Besides  these  there  is  a  paste,  in  which  minute 
scales  of  secondary  mica  and  needles  of  rutile  are  the  recog- 
nizable elements.  The  shales  of  the  South  Wales  coal-field3 
were  found  to  present  similar  characters,  though  much  ob- 
scured by  organic  pigment.  A  considerable  amount  of  clastic 
muscovite,  and  occasionally  biotite,  remains  with  the  quartz- 
granules,  and  the  paste  of  newly-formed  micaceous  material 
has  the  usual  rutile-needles. 

The  Culm-measure  shales  of  Bude  in  Cornwall4  are  derived 
from  the  waste  of  granite  (in  part  with  tourmaline)  and 
crystalline  schists.  They  appear  to  have  undergone  more 
change  in  situ  than  the  preceding. 

The  Cambrian  roofing-slates  of  North  Wales  represent  a 
more  advanced  stage  of  secondary  change,  both  structural  and 
mineralogical.  They  possess  a  strong  cleavage-structure,  pass- 
ing indifferently  through  the  layers  of  original  deposition,  and 
the  more  altered  of  them  have  the  glossy  aspect  of  fine-textured 
phyllites,  in  which  little  trace  of  any  clastic  structure  survives. 
Detrital  granules  of  quartz  and  felspar  may  be  seen,  but  biotite 
is  wanting,  though  little  patches  of  epidote  perhaps  represent 
it.  "The  base  and  main  constituent  of  all  these  slates  is  a 

1  For  a  description  of  various  American  clays  see  Merrill,  Guide  to 
Collections  in    Applied    GeoL,   Nonmetallic   Minerals   (1901),   325-328, 
pi.  15-17. 

2  G.  M.  1890,  264-273. 

*  Hutchings,  G.  M.  1896,  310. 

4  McMahon,  G.  M.  1890,  108-113  ;  Hutchings,  ibid.  188. 


CORNISH   AND  ARDENNES   SLATES.  245 

fine-grained  mica,  mostly  lying  flat  in  the  plane  of  cleavage 
of  the  rock,"  and  ru tile-needles  are  usually  abundant.  The 
red  and  purple  slates  contain  numerous  scales  of  red  mica- 
ceous haematite,  probably  representing  the  limonite  of  less 
altered  deposits.  A  number  of  specimens  of  slates,  Cambrian 
and  Ordovician,  from  this  region  have  been  described  by 
Mr  Hutchings1. 

The  Devonian  slates  of  Cornwall  (Tintagel,  etc.)  are  de- 
scribed by  the  same  author2  as  having  suffered  more  alteration 
(ascribed  to  dynamic  metamorphism)  than  the  Welsh  rocks. 
They  have  no  clastic  quartz,  felspar,  or  biotite,  and  indeed 
some  very  small  zircons  seem  to  be  the  only  derived  consti- 
tuents left  unaltered.  The  main  mass  of  the  rock  is  of  fine 
sericitic  mica,  the  majority  of  the  minute  flakes  being  parallel 
to  the  cleavage  of  the  rock.  Minute  needles  of  rutile  are 
very  abundant.  Another  very  common  mineral  is  micaceous 
ilmenite  in  flakes  about  '002  inch  in  diameter.  This  is  either 
opaque  or  transparent,  with  a  deep  brown  colour,  and  some- 
times encloses  characteristic  skeletons  of  rutile  (sagenite). 
Other  constituents  of  some  of  these  slates  are  secondary  quartz, 
calcite,  chlorite,  ottrelite,  garnet,  etc. 

The  Cambrian  phyllites  of  the  Ardenne  have  been  carefully 
examined  by  Prof.  Renard3,  who  finds  that  the  rocks  have  been 
completely  reconstituted  in  place.  The  chief  mineral  is  usually 
a  colourless  sericitic  mica,  its  flakes  having  a  general  parallel- 
ism with  the  cleavage  or  schistosity  of  the  rock.  This  and 
quartz  usually  constitute  the  principal  part  of  the  bulk,  and  a 
green  chlorite  is  also  abundant.  Needles  of  rutile  and  often 
of  tourmaline  lie  in  general  parallel  to  the  cleavage.  The 
violet  phyllites  have  micaceous  haematite  ('  oligiste ') ;  in  others 
micaceous  ilmenite  occurs,  with  interpositions  of  sagenite. 
Other  minerals  found  in  particular  rocks  are  magnetite  and 
pyrites,  a  manganese-garnet  (spessartine)  in  minute  crystals, 
ottrelite,  zircon,  carbonaceous  matter,  etc.  The  magnetite  in 
the  '  phyllade  aimantifere '  was  formed  before  the  cleavage  of 

1  Pr.  Liverp.  G.  S.  (1900)  viii,  464-471,  pi.  i,  and  (1901)  ix,  113,  114, 
pi.  vi,  figsE,F. 

2  G.  M.  1889,  214-220  ;  1890,  317-320. 

3  G.  M.  1883,  322-324  (Abstract). 


246  EXAMPLES   OF   CLEAVAGE-STRUCTURES. 

the  rock,  and  is  surrounded  by  the  curious  eyes  of  chlorite  and 
quartz  already  referred  to.  The  ottrelite  was  formed  subse- 
quently to  the  cleavage  of  the  rocks  which  contain  it,  and  its 
flakes  do  not  lie  parallel  to  the  cleavage-planes. 

American  phyllites  exhibiting  all  the  salient  characteristics 
have  been  described  from  the  Piedmont  Plateau  in  Maryland1, 
from  the  Lisbon  group  in  New  Hampshire2,  and  from  Coanicut 
Island,  R.I.3  A  fuller  account,  with  coloured  plates,  has  been 
given  by  Nelson  Dale4  of  the  phyllites  of  the  slate-belt  of  New 
York  and  Vermont.  These  rocks  consist  of  sericitic  mica 
(about  40  per  cent.),  quartz,  and  chlorite,  with  carbonates, 
pyrites,  sometimes  haematite,  zircon,  and  tourmaline,  and  in 
all  cases  minute  needles  of  rutile. 

Of  ordinary  slaty  cleavage  good  illustrations  are  afforded 
by  the  Cambrian  and  Ordovician  in  North  Wales,  the  De- 
vonian in  Cornwall,  and  some  other  British  Palaeozoic  rocks. 
Some  of  these  (Llanberis  Slates)  exhibit  the  deformation  of 
originally  spherical  spots.  Various  kinds  of  'eyes'  about 
enclosed  pyrites  crystals  may  be  seen  at  Penrhyn  (fig.  55,  A), 
Snowdon,  Blaenau  Ffestiniog,  Whitesand  Bay,  etc.,  and  in 
the  Cowal  district  of  Argyllshire 5.  Special  structures  of  the 
nature  of  false  cleavage  may  be  examined  in  the  Skiddaw 
Slates  of  the  Eden  valley  (Brownber,  near  Appleby,  fig.  55,  B\ 
and  of  Snaefell  in  the  Isle  of  Man,  in  the  debatable  rocks  of 
the  Start  in  South  Devon6  and  in  the  remarkable  'gnarled' 
beds  of  Amlwch  in  Anglesey  and  of  Aberdaron,  etc.,  in  the 
west  of  Caernarvonshire.  These  last  shew  very  beautifully  all 
the  characteristic  structures  of  'mountain-building,'  on  a  small 
scale,  and  such  rocks  afford  from  this  point  of  view  an  interest- 
ing study.  Prof.  Heim,  in  a  figure7  illustrating  the  passage  of 
an  overfold  into  an  overfault  by  the  obliteration  of  the  '  middle 

1  G.    H.   Williams,    Bull.   G.   S.  Amer.   (1891)   ii,  305-307 ;   Diller, 
317-320. 

2  Diller,  321-323. 

3  Pirsson,  A.  J.  S.  (1893)  xlvi,  376,  377. 

4  19th  Ann.  Rep.  U.  S.  Geol.  Sur.   (1899)  part  HI,  226-260,  265,  288- 
290,  pi.  xxxv-xxxix. 

5  Clough,  Mem.  Geol.  Sur.  Scot.,  Geol.  of  Cowal  (1897),  57,  80. 

6  G.  M.  1889,  214-220 ;  1890,  317-320. 

7  Mechanismus  der  Gebirgsbildung  (1883),  pi.  xv,  fig.  14. 


MICROSCOPIC   OVERTHRUSTING.  247 

limb,'  gives  for  the  scale  '-f2  to  TWOO-  °/  natural  size.' 
Perhaps  the  best  British  districts  for  studying  the  various 
forms  of  false  cleavage  are  the  Isle  of  Man,  where  the  Skiddaw 
Slates  exhibit  a  great  variety  of  interesting  structures,  and 
the  Cowal  district  of  Argyllshire1. 

1  dough,  I.e.,  7-29. 


CHAPTER  XVIII. 

CALCAREOUS  EOCKS. 

THE  different  kinds  of  limestones  (Fr.  calcaire,  Ger.  Kalk- 
stein),  consisting  of  carbonate  of  lime  with  various  impurities 
or  foreign  materials,  are  almost  all  in  great  measure  of  organic 
origin.  The  hard  parts  of  calcareous  organisms  are  composed 
of  calcite  or  aragonite1,  or  both,  with  a  small  quantity  of 
phosphate,  etc.  It  will  be  seen  that  aragonite  is  always  the 
unstable  form  of  carbonate  of  lime,  and  tends  to  be  converted 
into  the  stable  form,  calcite. 

The  impure  calcareous  rocks  may  include  a  considerable 
amount  of  non-calcareous  material;  either  sand-grains  (cal- 
careous grit)  or  finer  detritus  (argillaceous  limestone,  marl) 
or  volcanic  debris  (calcareous  tuff). 

With  the  limestones  must  be  classed  those  rocks  in  which 
dolomite  takes  the  place  of  calcite.  These  are  called  dolomite- 
rocks  or  dolomites,  the  name  dolomitic  limestone  or  magnesian 
limestone  being  more  correctly  applied  to  rocks  in  which  both 
minerals  are  well  represented.  Many  dolomitic  rocks  can  be 
proved  to  have  originated  from  ordinary  limestones,  the  mag- 
nesia which  replaced  part  of  the  lime  having  been  derived  from 
some  external  source. 

We  shall  also  briefly  notice  certain  other  rocks,  such  as 
some  bedded  iron-stones,  which  are  genetically  connected  with 
the  limestones,  and  some  siliceous  rocks  of  like  origin. 

1  According  to  Miss  Kelly  the  substance  which  has  been  regarded  as 
nragonite  is  in  reality  a  third  form  of  lime  carbonate,  which  she  names 
'conchite';  M.  M.  (1900)  xii,  363-370. 


CALCAREOUS   ALG.E.  249 

Much  valuable  information  concerning  limestones  is  con-, 
tained  in  Dr  Sorby's  Presidential  Address  to  the  Geological 
Society1,  while  British  limestones  from  various  horizons  have 
been  studied  by  several  other  observers2. 

Organic  fragments.  Most  of  the  fragments  of  calcare- 
ous organisms  that  form  part  of  rocks  have  something  in  their 
mineral  nature,  their  structure,  or  their  mode  of  preservation, 
that  enables  us  to  refer  them  to  their  proper  order  or  class,  or 
at  least  sub-kingdom. 

Among  vegetable  organisms,  the  calcareous  alga?  figure 
largely  in  the  deposits  now  forming  round  coral-islands3  and 
to  a  less  extent  in  some  deep-sea  deposits,  while  the  equivalents 
of  these  rocks  are  recognized  among  the  Tertiary  and  Recent 
strata  in  various  parts  of  the  world4;  e.g.  the  Lithothamnion 
Limestone  and  Leitha  Limestone  of  the  Vienna  basin  (compare 
fig.  56).  Calcareous  algae  are  concerned  in  the  formation  of 
some  modern  oolitic  accumulations,  and  Girvanella,  which 
figures  largely  in  association  with  oolitic  structure  in  rocks 
of  various  ages,  is  perhaps  a  vegetable  organism  ;  while  the 
peculiar  algous  flora  of  hot  springs  is  instrumental  at  the 
present  day  in  producing  certain  deposits  of  travertine  (Mam- 
moth Hot  Springs6).  The  part  played  by  algae  in  the  formation 
of  some  of  the  older  limestones,  such  as  the  Alpine  Trias,  seems 
to  be  of  considerable  importance 6.  In  some  fresh- water  lime- 


1  Q.  J.  G.  S.  (1879)  xxxv,  Proc.  56-95.     On  calcite  and  aragonite 
organisms,  see  also  Cornish  and  Kendall,  G.  M.  1888,  66-73  ;  Kendall, 
Eep.  Brit.  Ass.  for  1896,  789-791. 

2  See  especially  several  papers  by  Wethered,  Q.  J.  G.  S.  (1888-1893) 
xliv-xlix,  etc.  ;  Jukes-Browne  and  Hill  on  Chalk,  etc.,  ibid.  (1887-1889) 
xliii-xlv. 

3  See  Murray  and  Eenard,  '  Challenger '  Report  on  Deep-Sea  Deposits 
(1891),  pi.  xm,  xrv. 

4  Murray,  Scott.  Geog.  Mag.  (1890)  vi,  pi.  i  (Malta)  ;  Hill,  Q.  J.  G.  S. 
(1891)  xlvii,  243-248,  pi.  ix  (Barbados)  ;  Lister  (and  Murray),  ibid.  602, 
603  (Tonga  Is.)  ;  Gregory,  Q.  J.  G.  S.  (1892)  xlviii,  538-540  (Trinidad)  ; 
Hinde,   Q.  J.  G.  S.  (1893)  xlix,  230,  231  (New  Hebrides).    For  good 
figures  shewing  the  structures  of  Lithothamnion  and  other  calcareous 
algse  see  Bothpletz  in  Zeits.  deuts.  geol.  Ges.  (1891)  xliii,  pi.  xv-xvii. 

5  Weed  in  9th  Ann.  Eep.  U.  S.  Geol.  Sur.  (1890)  642-645,  etc. 

6  Cf.  Seward,  Science  Progress  (1894),  ii,  10-26. 


250  FORAMINIFERA. 

Atones,  such  as  those  of  Bembridge  and  of  Purbeck1,  Char  a 
is  sometimes  an  important  element. 


B 


FIG.  56.     RECENT  OKGANIC  LIMESTONES,  COMPOSED  LAKGELY  OF  CALCAREOUS 
ALG.E,  EUA,  TONGA  ISLANDS  ;    x  20. 

A  is  a  characteristic  section  of  Lithothamnion  [1271].  B  shews 
foraminifera  and  fragments  of  algae  in  a  recrystallized  calcareous  matrix 
[1269]. 

The  tests  of  calcareous  foraminifera  commonly  occur  entire, 
and  are  readily  recognized,  though  in  some  cases  the  chambers 
become  detached  (Globigerina,  fig.  65).  The  material  is  calcite 
or  aragonite  in  different  forms  (answering  to  the  division  into 
Vitrea  and  Porcellanea  of  some  authors),  and  probably  the 
latter  have  been  largely  destroyed  in  some  older  limestones. 
Foraminifera  occur  in  many  shallow- water  limestones2,  and 
make  up  a  large  part  of  the  so-called  coral-limestones3,  besides 
forming  the  bulk  of  extensive  deep-sea  deposits.  The  Nummu- 
litic  Limestone  is  a  well-known  instance  of  a  rock  composed 

1  Wethered,  Pr.  Cottesio.  F.  N.  Club  (1891),  x,  101,  102,  with  plate. 

2  See,  e.g.,  Guppy,  Tr.  Roy.  Soc.  Edin.  (1885)  xxxii,  pi.  CXLV,  figs.  1, 
4  (Solomon  Is.) ;   Jennings,  G.  M.  1888,  pi.  xiv  (Orbitoidal  Limestone 
of  Borneo). 

3  See  Guppy,  The  Solomon  Islands,  Geology,  etc.  (1887),  73-76  ;  and 
Tr.  Roy.  Soc.  Edin.  (1885)  xxxii,  545-581 ;  Lister  (and Murray),  Q.  J.  G.  S. 
(1891)  xlvii,  602-604  (Tonga  Is.). 


CORALS  :    ECHTNODERMS.  251 

largely  of  foraminifera.  Other  examples  are  the  Alveolina 
or  Miliolite  Limestone  of  Mixen  Rocks  near  Selsea  and  the 
Saccamina  Limestone  of  Northumberland. 

The  interior  of  a  foraminiferal  test  may  be  filled  in  by 
crystalline  calcite,  often  with  such  a  radial  arrangement  of 
fibres  as  to  give  a  very  perfect  black  cross  in  each  chamber 
when  examined  between  crossed  nicols.  In  many  modern 
sediments1  formed  near  a  continental  shore-line  the  chambers 
are  occupied  by  a  deposit  of  green  glauconite,  which,  by  the 
removal  of  the  calcareous  test,  may  be  left  in  the  form  of  casts ; 
and  this  seems  to  be  the  usual  mode  of  origin  of  glauconite- 
sands,  such  as  are  found  at  various  geological  horizons2. 

The  true  corals  consist,  according  to  Dr  Sorby,  of  little 
fibres,  or  in  some  cases  granules,  of  aragonite  ;  but  it  appears 
that  calcite  enters  into  the  composition  of  some  forms.  Mr 
Kendall  states  that,  while  almost  all  the  reef-building  forms  have 
aragonite  skeletons,  all  the  deep-sea  corals  examined  by  him 
are  of  calcite.  Of  the  Rugosa  some  consist  largely  of  calcite 
fibres  roughly  parallel  to  the  outlines  of 'the  several  parts  of 
the  skeleton,  while  the  mode  of  preservation  of  others  seems 
to  indicate  that  they  were  composed  largely  of  aragonite.  The 
so-called  coral-rock,  coral-sand,  and  coral-mud  of  Recent  strata 
and  of  deposits  now  forming  often  consist  largely  of  calcareous 
algse  or  foraminiferal  tests,  but  some  are  of  almost  pure  corals 
and  coral  fragments.  Among  older  rocks  having  this  consti- 
tution may  be  mentioned  parts  of  the  Mountain  Limestone 
and  the  Coral  Oolite  and  certain  Devonian  limestones  of  South 
Devon  (Torquay  and  Plymouth). 

The  hard  parts  of  echinoderms  have  an  unmistakable 
appearance.  Each  element  (plate  or  joint)  behaves  optically 
as  a  single  crystal  of  calcite,  the  larger  ones  shewing  the 
characteristic  cleavage.  The  organic  nature  is  indicated  only 
by  the  external  form,  internal  canals,  etc.  Spines  of  echinoids, 

1  Murray  and  Benard,  Deep-Sea  Deposits  (1891),  pi.  xxiv,  xxv. 

2  See,  e.g.,  Murray,  Scott.  Geog.  Mag.  (1890)  vi,  464,  465,  pi.  n,  fig.  2 
(Malta)  ;  Gregory,  Q.  J.  G.  S.  (1892)  xlviii,  540  (Trinidad).     Cf.  Sollas, 
Q.  J.  G.  S.  (1872)  xxviii,  399  (Cambridge  Greensand),  and  Hume,  ibid. 
(1897)  liii,  569-571  (U.  G.  S.  of  Woodburn,  Antrim). 


252 


CRUSTACEA:    POLYZOA. 


joints  of  the  stems  of  crinoids,  etc.,  may  be  distinguished  by 
their  size  and  outline  (fig.  57). 


FIG.  57.    LIASSIC  LIMESTONE,  SKYE  ;    x  15  : 

shewing  joints  of  crinoid  stems  (Pentacrinus)  cut  longitudinally  (cr), 
and  transversely  (cr'),  each  consisting  of  a  single  crystal  of  calcite  ;  also 
part  of  a  brachiopod  shell  (Rhynchonella,  br),  with  its  characteristic 
lamellar  structure.  The  matrix  is  a  recrystallized  calcite  mosaic  enclosing 
numerous  detrital  grains  of  quartz  and  flakes  of  muscovite  [1791]. 

The  structure  of  the  hard  parts  of  Crustacea  is  also  fairly 
constant  and  quite  different  from  the  preceding.  The  shell  is 
built  of  fibres  of  calcite  set  everywhere  perpendicular  to  the 
surface,  the  optic  axis  of  each  fibre  coinciding  with  its  length. 
The  general  outline  suffices  to  distinguish,  e.g.,  between  ento- 
mostracan  tests  (abundant  in  many  limestones)  and  fragments 
of  trilobites  (fig.  58,  A). 

Both  calcite  and  aragonite  enter  into  the  composition  of 
the  polyzoa,  and  in  some  genera,  according  to  Messrs  Cornish 
and  Kendall,  the  two  occur  in  separate  layers,  the  aragonite 
layer  being  in  this  case  the  outer  one. 

The  shells  of  brachiopods  are  wholly  of  calcite,  with  a  cha- 
racteristic structure.  "  They  are  made  up  of  laminae,  consisting 


BRACHIOPODS:  LAMELLIBRANCHS.        253 

of  flattened  fibres  or  prisms,  often  passing  along  more  or  less 
parallel  to  one  another  over  a  considerable  area,  but  mixed  up 


B 


Fro.  58.     CARBONIFEROUS  LIMESTONE,  CLIFTON,  BRISTOL  ;    x  20. 

A  shews  a  portion  of  a  trilobite  with  the  characteristic  structure  of 
the  Crustacea  [981].  B  polyzoa  replaced  by  opaque  limonite,  mixed  with 
silica,  in  a  matrix  of  coarsely  crystalline  calcite  [972]. 

with  other  systems  which  cross  them  at  various  angles. "  These 
laminae  lie  oblique  to  the  surface  of  the  shell,  and  the  indi- 
vidual fibres  do  not  give  strictly  straight  extinction  (fig.  57). 
The  'perforations'  of  some  brachiopod  shells  can  be  seen,  but 
they  are  not  a  characteristic  feature. 

The  shells  of  lamellibranchs  have  more  than  one  type  of 
structure.  In  some  ostreid  genera  (Ostrea,  Pecten,  Gryphsea, 
Inoceramus)  the  whole  is  of  calcite  in  irregular  flattened  fibrous 
plates,  producing  a  structure  not  unlike  that  of  brachiopods. 
The  shells,  however,  are  usually  of  stouter  build,  and  they 
tend  to  break  up  into  their  component  prisms  or  fibres,  which 
are  often  found  detached,  e.g.,  Inoceramus  in  the  Chalk.  On 
the  other  hand,  most  lamellibranch  shells  consist  originally  of 
aragonite,  and  are  commonly  preserved  only  as  casts  in  calcite 
mosaic  (fig.  59).  In  some  genera  (Pinna,  Mytilus,  Spondylus) 


254 


GASTEROPODS  :  CEPHALOPODS. 


there  is,  according  to  Dr  Sorby,  an  inner  layer  of  aragonite 
protected  by  an  outer  layer  of  calcite. 


oo 


FIG.  59.     OOLITIC  LIMESTONE,  MILLEPORE  OOLITE,  WIIAIUUM, 
EAST  YORKSHIRE  ;    x  20  : 

shewing  oolitic  grains  (oo)   and  chips  of  lamellibranch  shells  (s)    in   a 
matrix  which  has  recrystallized  as  a  mosaic  of  clear  calcite  [1794]. 

Most  gasteropods  have  shells  wholly  of  aragonite,  which  is 
readily  replaced  by  a  mosaic  of  crystalline  calcite.  In  a  few 
cases,  however,  e.g.  Scalaria,  the  whole  is  of  calcite  (Cornish 
and  Kendall).  Others  have  a  layer  of  aragonite  covered  by 
a  layer  of  calcite  :  either  the  former  (Murex)  or  the  latter 
(Purpura)  may  form  the  bulk  of  the  shell. 

Of  the  cephalopoda,  the  shells  of  Nautilus  and  the  am- 
monites were  originally  of  aragonite,  but  the  aptychi  of  the 
ammonites  were  of  calcite.  The  belemnites  had  the  guard  of 
calcite,  with  a  characteristic  radial  arrangement  of  fibres 
about  an  axis,  but  the  phragmocone  was  of  aragonite. 

The  tests  of  pteropoda  consist,  according  to  Mr  Kendall, 
of  aragonite,  and  may  sometimes  be  recognized  by  their  form  in 
sections.  Exceptionally  they  form  the  main  constituent  of  a 
limestone,  and  'pteropod  ooze'  is  one  of  the  deep-sea  deposits 
now  forming  in  some  parts  of  the  ocean. 


OOLITIC   STRUCTURE.  255 

Oolitic  structure1.  Many  shallow- water  limestones,  of 
all  geological  ages,  contain  little  spheroidal  grains  built  up  of 
successive  coats  of  calcareous  material,  and  these  may  be  so 
numerous  as  to  make  up  the  chief  bulk  of  the  rock.  Such 
rocks  are  called  oolitic  limestones,  oolites,  or  roestone  (Ger. 
Rogenstein).  For  the  coarser  types,  in  which  the  grains  may 
reach  the  size  of  peas,  and  are  often  of  rather  irregular  or 
flattened  form,  the  name  pisolite  (Ger.  Erbsenstein)  is  used. 

In  addition  to  the  concentric-shell  arrangement,  there  is 
often  a  more  or  less  evident  radial  structure  in  each  grain, 
and  closer  examination  shews  that  the  minute  elements  which 
build  up  the  successive  layers  are  set  in  some  cases  radially,  in 
other  cases  parallel  to  the  layers. 

As  a  result  of  either  of  these  arrangements  an  oolitic  grain, 
examined  in  section  between  crossed  nicols,  should  give  a  black 
cross  comparable  with  that  observed  in  the  spherulites  of 
igneous  rocks.  Owing  to  the  departure  from  true  sphericity, 
the  admixture  of  granular  material  not  sharing  the  definite 
orientation  described,  and  the  effect  of  iron-staining  and  other 
secondary  changes,  an  accurate  black  cross  is  not  seen  in  every 
case. 

The  concentric  layers  have  been  formed  upon  a  nucleus, 
which  may  be  a  chip  of  shell  or  other  organic  body,  a  quartz- 
granule,  or  merely  a  pellet  of  fine  calcareous  mud.  Similar 
coatings  are  often  to  be  seen  upon  fragments  of  shell,  etc.,  too 
large  to  be  built  up  into  round  grains.  Sometimes  an  oolitic 
grain  has  been  broken  and  the  separated  fragments  subse- 
quently coated  with  fresh  layers  of  calcareous  deposit ;  or 
again  two  or  three  contiguous  grains  may  be  enveloped  in  one 
mantle  and  become  a  compound  grain. 

Oolitic  grains  differ  as  regards  their  material  (calcite  or 
aragonite),  the  orientation  of  their  minute  elements  (radial  or 
tangential),  the  presence  or  absence  of  finely  granular  calcare- 

1  On  the  oolitic  structure  and  its  significance  see  Sorby's  Prcsid. 
Address,  I.  c. ;  also  Teall  in  Mem.  Geol.  Sur.,  Jurassic  Rocks  of  Britain, 
vol.  iv,  pp.  8-12,  pi.  i,  n,  1894 ;  Wethered  (papers  cited).  Various  types 
of  oolitic  grains  are  described  and  figured  by  Harris,  Pr.  Geol.  Ass.  (1895) 
xiv,  59-79,  pi.  m,  iv. 


256  VARIETIES   OF   OOLITIC   GRAINS. 

pus  matter  without  special  orientation,  or  of  impurities,  and 
in  other  respects.  One  common  type1,  exemplified  in  many 
British  limestones,  has  well-marked  concentric  shells,  each  of 
which  consists  largely  of  minute  calcite  prisms  or  fibres  set 
radially.  There  may  or  may  not  be  an  evident  radial  structure 
in  the  grain  as  seen  in  a  thin  slice.  The  black  cross  seen  in 
polarized  light  is  often  imperfect  or  vaguely  defined. 

Another  type  is  illustrated  by  the  so-called  Sprudelstein  of 
the  Carlsbad  hot  springs2.  Here  there  are  well-marked  con- 
centric shells  but  no  radial  structure.  The  material  is  arago- 
nite  and  the  minute  elements  are  set  mainly  tangentially  to  the 
concentric  layers.  This  gives  a  well-defined  black  cross.  Dr 
Sorby  found  recent  oolites  from  Bahama3  and  Bermuda  to  have 
a  similar  constitution,  but  with  some  unoriented  granular 
material,  and  he  observed  the  same  in  the  Bembridge  Lime- 
stone of  the  Isle  of  Wight. 

It  is  impossible  to  say  with  certainty  to  what  extent 
aragonite  oolitic  grains  have  once  been  represented  in  our 
older  rocks.  In  numerous  instances  the  present  structure 
of  the  grains  shews  that  they  have  been  recrystallized.  They 
often  consist  of  crystalline  calcite,  either  in  a  mosaic  or  in 
wedges  with  a  rough  radial  arrangement.  In  some  cases  there 
is  an  eccentric  radial  structure,  as  if  the  recrystallization  had 
started  at  one  or  more  points  on  the  circumference  of  the 
grains. 

It  is  a  somewhat  difficult  question  how  far  the  original 
structure  of  the  different  types  of  oolitic  grains  is  due  on  the 
one  hand  to  mechanical  aggregation  or  on  the  other  to  crys- 
tallization, and  it  further  appears  that  organic  agency  may 
often  have  played  an  important  part.  The  Carlsbad  Sprudel- 
stein, the  calcareous  sand  of  Salt  Lake,  and  other  modern 
oolites  seem  to  be  connected  with  lime-secreting  algae ;  while 
Mr  Wethered4  finds  the  problematical  organism  Girvanella  in 

1  Cohen  (3),  pi.  LXIII,  figs.  2,  3. 

2  Harris,  L  c.,  pi.  in,  fig.  9. 

s  Ibid.  figs.  6-8  and  pp.  67-70. 

4  See  papers  cited  below,  but  especially  Q.  J.  G.  S.  (1895)  li,  196-206, 
pi.  vii,  where  the  organic  theory  is  extended  to  oolitic  limestones  in 
general :  also  Proc,  Cotteswold  Nat.  field  Clnb,  1895-6. 


MATRIX   OF   LIMESTONES.  257 

many  oolitic  rocks  of  various  ages.  It  is  well  seen  encrust- 
ing the  successive  layers  of  large  pisolitic  grains  in  such  a 
rock  as  the  Pea  Grit  of  Cheltenham,  and  again  in  some  oolites, 
e.g.  Wenlock  Limestone  (fig.  60). 


Fio.  60.     OOLITIC  GRAIN  FROM  THE  WENLOCK  LIMESTONE,  LONGHOPE  ;    x  6. 

The  concentric  coats  are  built  up  largely  of  the  interlacing  tubes  of 
Girvanella.  [This  figure  was  kindly  furnished  by  Mr  Wethered.] 

Matrix  of  limestones.  Recognizable  fragments  of 
organisms,  together  with  oolitic  grains,  if  present,  may  make 
up  a  variable  part  or  even  the  chief  bulk  of  a  limestone.  The 
remainder,  in  rocks  which  have  suffered  no  important  secondary 
changes,  consists  of  a  calcareous  mud  in  which  the  fragments 
(and  oolitic  grains)  are  embedded.  This  finely  divided  material 
is  mostly  carbonate  of  lime,  and  must  be  in  great  measure 
derived  from  the  attrition  and  disintegration  of  calcareous 
organisms,  though  chemical  deposition  may  perhaps  play  some 
part,  and  material  may  be  furnished  by  the  degradation  of 
older  limestones.  Iron-compounds  often  occur  as  an  impurity, 
producing  a  yellow  or  brown  stain  by  oxidation.  Fine  sand  of 
detrital  origin  is  often  present  in  shallow-water  limestones, 
and  may  be  abundant  (calcareous  grits).  Similarly,  an  admix- 
ture of  argillaceous  matter  gives  rise  to  argillaceous  limestones 
and  calcareous  marls,  or  by  the  presence  of  volcanic  detritus 

H.  P.  17 


258          RECRYSTALLIZED  MATRIX   OF   LIMESTONES. 

and  ashes  the  rock  becomes  a  calcareous  tuff.  In  some 
argillaceous  limestones,  such  as  those  of  the  English  Lias,  it 
is  probable  that  much  even  of  the  calcareous  matter  is  of 
detrital  origin1. 

In  many  limestones,  and  especially  those  belonging  to  the 
older  formations,  the  original  finely  divided  calcareous  matter 
has  been  partially  or  wholly  recrystallized  into  a  granular 
calcite-mosaic  of  fine  or  sometimes  comparatively  coarse  tex- 
ture. Crystalline  limestones  or  marbles  are  thus  formed  without 
any  special  conditions  of  the  kind  usually  implied  in  the  term 
metamorphism.  The  recrystallization  seems  to  originate  at 
certain  points  in  the  mass  and  spread.  The  process  has  a 
purifying  effect,  and  ferruginous  impurities  often  appear  as 
if  pushed  before  it  to  collect  in  particular  patches.  The 
recrystallized  carbonate  of  lime  is  always  calcite,  aragonite 
being  converted  in  the  process  to  the  stabler  form.  In  such  a 
crystalline  matrix  casts  after  aragonite  shells  may  usually  be 
recognized  by  a  rather  coarser  mosaic  and  by  a  thin  film  of 
impurities  marking  the  original  outline,  even  when  they  are 
not  coated  in  oolitic  fashion  (fig.  59). 

The  recrystallized  calcite  usually  forms  a  more  or  less  finely 
granular  mosaic  in  the  interstices  between  the  organic  frag- 
ments, oolitic  grains,  etc.  In  some  cases,  however,  the  individual 
crystal-grains  of  calcite  are  of  large  size,  so  as  to  enclose  numer- 
ous oolitic  granules,  shell-fragments,  etc.,  thus  giving  a  structure 
like  the  ophitic  and  poecilitic  in  some  igneous  rocks.  This  has 
been  remarked  by  Mr  Teall  in  some  of  the  oolitic  building-stones 
of  the  Lincolnshire  Limestone  (Barnack,  Ketton,  Ancaster). 
An  analogous  structure  has  already  been  noted  above  in  certain 
calcareous  grits  with  abundant  calcite  matrix,  the  Fontaine- 
bleau  Sandstone  affording  an  extreme  example. 

In  certain  coarse-textured  marbles  the  new-formed  calcite 
occurs  partly  as  a  crystal  outgrowth  of  fragments  of  crinoids, 
etc.,  comparable  with  the  quartz-cement  of  many  quartzites 
(Clifton). 

i  Woodward,  Jurassic  Eocks  Engl  and  Wales,  vol.  iii  (1893),  27-32  ; 
cf  Sollas,  Q.  J.  G.  S.  (1879)  xxxv,  492  (limestones  in  0.  R.  S.  of  Cardiff 
district)  and  G.  M.  1900,  248-250. 


DEEP-SEA   OOZES.  259 

In  some  oolitic  limestones  the  original  matrix  has  been  in 
great  measure  removed  by  solution,  leaving  vacant  spaces 
between  the  oolitic  grains.  This  is  seen  in  some  of  the 
Ancaster  and  Ketton  building-stones,  belonging  to  the  Lin- 
colnshire Limestone1.  In  other  cases  the  oolitic  grains  are 
themselves  recrystallized  to  a  granular  mosaic2. 

The  quartz-sand,  etc.,  occurring  as  impurities  in  many 
limestones  can  be  easily  isolated  by  dissolving  the  rock  in 
dilute  acid,  and  sometimes  present  points  of  interest3.  Minute 
perfect  crystals  of  quartz  may  occur,  sometimes  evidently 
formed  by  secondary  outgrowth  from  detrital  quartz-grains 
(Clifton). 

Deep-sea  calcareous  deposits.  Beyond  the  broad  belt 
of  deposits  now  forming  along  the  continental  coast-lines  and 
deriving  their  material  in  some  degree  from  the  waste  of  the 
land  and  from  shallow-water  organisms,  and  apart  too  from 
the  special  accumulations  forming  round  coral-  and  volcanic 
islands,  extensive  calcareous  deposits  are  found  covering  large 
areas  of  the  floor  of  the  deep  ocean  down  to  about  2800  fathoms. 
The  most  widely  spread  of  these  deposits  is  globigerina-ooze, 
consisting  largely  of  the  tests  of  Globigerina  and  other  foramin- 
ifera4,  together  with  a  smaller  proportion  of  other  organisms, 
such  as  siliceous  radiolaria,  and  some  non-calcareous  matter  of 
volcanic  origin.  Associated  with  the  foraminiferal  remains 
are  immense  numbers  of  very  minute  elliptic  disc-shaped  bodies, 
to  which  Prof.  Huxley  gave  the  name  coccoliths6.  These 
calcareous  discs  have  been  detached  from  the  surface  of 
certain  globular  organisms  named  coccospheres,  referred  to 
the  algae.  The  coccoliths  have  a  diameter  of  *0002  to  "0005 


1  On  this  and   some   other  north-country   Jurassic  limestones  see 
Naturalist,  1890,  300-304. 

2  Cohen  (3),  pi.  LXXIII,  fig.  4. 

3  Wethered  (Carboniferous),  Q.  J.  G.  S.  (1888)  xliv,  186-198  ;  (Inferior 
Oolite)  ibid.  (1891)  xlvii,  559-569. 

4  Murray  and  Benard,  Deep-Sea  Deposits  (1891),  pi.  xi,  figs.  1,  5,  6 ; 
xii ;  xv,  fig.  2. 

5  Ibid.,  pi.  xi,  figs.  3,  4.     See  also  Wallich,  Ann.  Mag.  Nat.  Hist. 
(1861)  ser.  3,  viii,  52-56,  and  on  the  coccoliths  of  the  Chalk  see  Sorby, 
ibid.,  193-200. 

17—2 


260  DOLOMITIZATION   OF   LIMESTONES. 

inch.  Associated  with  them  are  often  other  minute  bodies 
in  the  form  of  slender  rods  with  a  crutch-like  termination 
(rhabdolitbs).  Coccoliths  and  rhabdoliths  are  very  character- 
istic of  the  deep-sea  calcareous  deposits,  though  not  confined 
to  them. 

The  inorganic  residue  of  these  rocks  is  essentially  of 
volcanic  material  in  a  state  of  extremely  fine  division,  and 
corresponds  with  the  '  red  clay '  already  noticed. 

Various  foraminiferal  and  other  limestones  have  been  de- 
scribed among  Tertiary  and  Recent  strata  which  approximate, 
in  some  cases  very  closely,  to  the  essential  characters  of  true 
deep-sea  deposits1. 

Metasomatic  changes  in  limestones.  In  many  rocks 
which  may  be  assumed  to  have  been  once  ordinary  limestones, 
the  carbonate  of  lime  has  been  partly,  or  even  wholly,  replaced 
by  other  substances,  thus  producing  a  change  in  the  chemical 
composition  of  the  rock  (metasomatism).  The  most  common 
of  such  changes  is  that  in  which  calcite  is  converted  into 
dolomite  by  the  replacement  of  half  its  lime  by  magnesia 
(dolomitizatiwi).  It  seems  to  be  clearly  established  that 
calcite  and  dolomite  are  not  chemically  isomorphous  sub- 
stances, but  each  has  its  own  definite  composition.  The 
molecular  ratio  CaO  :  MgO  in  dolomite  is  always  unity,  and  a 
higher  ratio  in  the  bulk-analysis  of  a  dolomitic  rock  indicates 
a  mixture  of  dolomite  and  calcite. 

In  the  finely  granular  mosaic  which  such  rocks  often 
present  it  may  be  difficult  to  distinguish  the  two  minerals 
from  one  another  without  chemical  tests2.  One  criterion  is 
the  much  stronger  tendency  of  dolomite  to  develope  crystal 
outlines3,  always  those  of  the  primitive  rhombohedron  (fig.  61). 
In  coarse-grained  rocks  the  more  marked  cleavage-traces  of 
calcite  and  the  frequency  in  it  of  lamellar  twinning4  help  to 

J  E.g.  Hill,  Q.  J.  G.  S.  (1892)  xlviii,  179  (Barbados). 

2  Lemberg  has  given  a  microchemical  test  applicable  to  rock-slices  ; 
M.  M.  (1889)  viii,  166  (Abstr.). 

3  Cf.  Wethered,  Q.  J.  G.  S.  (1892)  xlviii,  fig.  on  p.  381 ;  Eutley,  ibid. 
(1894)  1,  pi.  xix,  figs.  5,  6. 

4  Cohen  (3),  pi.  xxvii,  fig.  4  (Carrara  marble). 


DOLOMITIZATION   OF   LIMESTONES. 


261 


distinguish  it  from  dolomite.     Again,  calcite  is  colourless  in 
slices,  while  dolomite  usually  (but  not  always)  has  a  yellow  or 


FlG.    61.      DOLOMITIZED   LIMESTONE   IN   UPPER    CoNISTON   LlMESTONE, 

SHAP  WELLS,  WESTMORLAND  ;    x  20, 

The  dolomite  is  here  in  good  rhombohedra  with  a  zonary  structure 
marked  by  inclusions  :  some  calcite  remains  as  a  clear  mosaic  [1616]. 

yellowish-brown  tint.  This  coloration  is  probably  due  to  iron. 
It  may  be  remarked  that  another  mineral  of  the  same  group  is 
sometimes  met  with,  viz. — chalybite,  or  siderite,  the  ferrous 
carbonate.  This  often  builds  little  rhombs  with  curved  out- 
lines. It  is  of  a  somewhat  deeper  brown  tint  than  dolomite, 
and  in  many  cases  encloses  little  opaque  specks  or  minute 
crystals  of  pyrites. 

Good  examples  of  more  or  less  perfectly  dolomitized  rocks 
occur  in  the  Durness  Limestones  of  Sutherland,  the  Bala  and 
Coniston  Limestones,  the  Devonian  of  Devonshire,  the  Car- 
boniferous Limestone  of  many  parts  of  England  and  Ireland, 
and  the  Permian  Magnesian  Limestone.  Among  foreign 
formations  may  be  mentioned  the  Alpine  Trias,  dolomitic 
rocks  attaining  a  great  development  in  the  southern  Tirol. 

In  many  cases  the  rocks  give  evidence  of  shrinkage  during 
the  process  of  dolomitization.  There  are  often  crevices  and 


262 


BEDDED   IRONSTONES. 


cavities,  which,  however,  may  be  filled  subsequently  by  an 
infiltration  of  calcite.  Some  dolomitized  oolitic  limestones 
shew  a  little  cavity  in  the  centre  of  each  oolitic  grain 
(Magnesian  Limestone  near  Hartlepool). 

Again,  certain  ironstones  have  evidently  been  formed1  by 
metasomatic  changes  from  limestones.  The  process  consists 
first  in  the  replacement  of  calcite  by  ferrous  carbonate  (chalyb- 
ite), and  further,  in  many  cases,  in  an  oxidation  of  the  latter, 
giving  rise  to  magnetite,  haematite,  or  limonite.  The  oolitic 
limestones  seem  to  be  specially  liable  to  this  kind  of  alteration, 
and  the  oolitic  grains  themselves  shew  the  most  advanced 
stage,  the  outer  part  of  each  grain  being  converted  into  mag- 


FIG.  62.    IRONSTONES. 

A.  Ironstone-band  in  Scarborough  Limestone,  Scarborough  ;  x  100, 
shewing  an  aggregate  of  minute  rhombs  of  chalybite,  often  enclosing 
nuclei  of  pyrites.  The  clear  grains,  of  which  two  are  shewn,  are  quartz 
[946].  B.  Oolitic  ironstone,  Claxby,  Lincolnshire ;  x  20.  Here  the 
oolitic  grains  are  transformed  to  limonite  ;  the  matrix  is  mostly  of 
chalybite,  but  has  undergone  in  patches  the  further  change  to  limonite 
[1591]. 

1  See  Sorby,  I.e.  pp.  54,  55  ;  Jndd,  Geol.  of  Rutland,  117-138  ; 
Hudleston,  Pr.  Geol.  Ass.  (1889)  xi,  117-127;  Cole  and  Jennings, 
Q.  J.  G.  S.  (1889)  xlv,  426,  427  ;  Teall  in  Mem.  Geol.  Sur.,  Jurassic  Rocks 
of  Britain,  vol.  iii,  p.  302 ;  vol.  iv,  pi.  n,  etc. 


BEDDED   IRONSTONES.  263 

netite  or  limonite,  while  the  matrix  of  the  rock  remains  as 
chalybite  or  in  part  calcite.  The  chalybite  matrix  is  fine- 
textured,  and  the  mineral  often  shews  imperfect  crystal  form, 
each  crystal  sometimes  enclosing  a  nucleus  of  decomposing 
pyrites  (fig.  62,  A ).  In  a  more  advanced  stage  of  change  patches 
of  limonite  replace  the  chalybite  of  the  matrix  (fig.  62,  B),  and 
even  calcite  shells  of  Pecten,  etc.,  are  converted  into  haematite 
or  limonite  (e.g.  the  Dogger  of  the  Peak  in  Yorkshire).  The 
oxidation  does  not  take  place  in  the  more  argillaceous  iron- 
stones, the  iron  remaining  there  in  the  form  of  carbonate. 
Valuable  oolitic  ironstones  are  worked  in  this  country.  That 
of  Rosedale  (Dogger)  is  magnetite,  the  Cleveland  Main  Seam ' 
(Middle  Lias)  shews  various  stages  of  transformation  and 
various  admixtures  of  earthy  matter,  the  Jurassic  ores  of 
Northampton  and  Rutland  have  specially  the  limonite  type 
of  alteration,  and  the  Neocomian  ores  of  Tealby  and  Claxby 
in  Lincolnshire  are  similar.  An  oolitic  ironstone  with  more 
gritty  impurities  occurs  at  Abbotsbury  and  Westbury  in  the 
Coralliari  group  of  the  Isle  of  Purbeck2. 

._,  The  best  known  bedded  ironstone  of  this  kind  in  America 
is  the  Clinton  ore,  which  occurs  in  the  Silurian  of  the  eastern 
states,  and  is  worked  at  Clinton,  N.Y.,  Birmingham,  Ala., 
and  elsewhere.  Some  beds  are  truly  oolitic,  while  others 
have  a  quasi-oolitic  appearance,  the  grains  being  rolled 
fragments  of  polyzoans  replaced  by  iron-oxide.  While  some 
difference  of  opinion  exists  as  regards  the  origin  of  the  ore, 
it  seems  probable  that  it  is,  at  least  in  great  part,  formed 
from  limestone3. 

If  the  grains  of  an  oolitic  iron-ore  be  dissolved  by  acid, 
each  leaves  a  shell  or  skeleton  of  silica  soluble  in  caustic  potash. 
This  silica  must  have  been  introduced  at  some  stage  of  the 
alteration  of  the  original  limestone.  A  similar  siliceous  skele- 
ton is  sometimes  found  in  the  grains  of  oolitic  limestones  where 

1  For  figures  of  this  and  other  oolitic  ironstones  see  Mem.  Geol.  Sur. , 
Jurassic  Rocks,  vol.  iv,  pi.  n. 

2  Strahan,  Mem.   Geol.  Sur.,  Geol.  I.  Purb.  (1898)  39 ;  Teall  in  Mem. 
Geol.  Sur.,  Jurassic  Eocks,  vol.  v  (1895),  324;   see  also  vol.  iv,  pi.  n, 
fig.  12. 

3  See  Foerste,  A.  J.  S.  (1891)  xli,  28,  29 ;  Kimball,  Amer.  Geol.  (1891) 
viii,  356,  357 ;  Smyth,  A.  J.  S.  (1892)  xliii,  487-496  ;  Diller,  138-140. 


264  SILICIFICATION   OF   LIMESTONES. 

no  ferruginous  replacement  has  taken  place,  or,  again,  silica 
may  more  or  less  replace  the  calcareous  matter  between  the 
grains1.  Although  silicification  is  perhaps  less  common  than 
some  of  the  other  metasomatic  changes  noticed  above,  it  is 
found  in  numerous  limestones  of  various  ages.  Sometimes 
the  replacement  of  carbonate  of  lime  by  silica  is  confined  to 
the  organic  remains,  but  in  other  cases  it  affects  the  whole 
body  of  the  rock  (e.g.  some  cherts).  Parts  of  the  Carboniferous 
limestones  of  Clifton  shew  examples  of  oolitic  grains  and 
organic  fragments  replaced  by  a  mixture  of  limonite  and 
silica.  Good  examples  of  cherts  formed  by  the  silicification 
of  limestone  (matrix  and  fossils  alike)  are  found  in  the  Port- 
land Beds  of  the  South  of  England. 

An  almost  purely  siliceous  rock  from  eastern  Pennsylvania2 
shews  a  beautiful  oolitic  structure,  each  little  sphere,  about 
'04  inch  in  diameter,  consisting  of  numerous  concentric  coats 
surrounding  a  nucleus,  and  the  interspaces  being  also  occupied 
by  silica.  Here  there  must  evidently  have  been  a  mole- 
cular replacement  of  carbonate  of  lime  by  silica,  and  indeed 
associated  rocks  shew  various  stages  of  partial  replacement. 
Some  cherts  in  the  Durness  Limestone  of  Sutherland  tell  the 
same  story,  the  oolitic  structure  being  still  discernible  (Stone- 
chrubie  near  Ichnadamff).  Similar  oolitic  cherts  occur  in  the 
Corallian  of  Yorkshire,  in  the  Portlandian  of  St  Alban's  Head3, 
and  in  the  Carboniferous  of  South  Wales4. 

Still  another  metasomatic  change  met  with  in  some 
calcareous  rocks  is  phosphatization.  This  usually  affects  some 
or  all  of  the  organic  remains,  or  phosphatic  nodules  are  formed 
having  fossils  of  various  kinds  as  nuclei.  The  phosphate  of 
lime  is  presumably  itself  derived  from  organic  bodies,  but  it  is 
not  clear  to  what  extent  it  has  been  supplied  contemporaneously 
with  the  deposit  which  contains  the  nodules.  Deposits  rich  in 
phosphate  occur  at  various  horizons  in  the  formations  of  this 

1  Chapman,  G.  M.  1893,  100-104  (Devonian,  Ilfracombe). 

2  Barbour  and  Torrey,  A.  J.    S.    (1890)  xl,  247-249,  with  figures. 
Similar  rocks  occur  at  several  localities  in  Missouri ;  Hovey,  ibid.  (1894) 
xlviii,  404,  405 ;  Harris,  Pr.  Geol  Ass.  (1895)  xiv,  78,  pi.  iv,  fig.  9. 

3  Teall,  Mem.   Geol.  Sur.,  Geol.  I.  Purbeck  (1898),  63,  and  Jurassic 
Rocks,  vol.  v  (1895),  186. 

4  Watts,  Mem.  Geol.  Sur.,  S.  Wales  Coalfield,  part  n  (1900),  36. 


CAMBRIAN   AND   ORDOVICIAN   LIMESTONES.  265 

country :  the  Cambridge  Greensand  may  be  taken  as  an  example, 
where  the  fossils  are  largely  phosphatized  and  also  serve  to 
some  extent  as  the  nuclei  of  nodules.  In  other  instances 
phosphate  of  lime  occurs  as  casts  of  foraminifera1  or  as  grains 
more  or  less  definitely  replacing  those  bodies2.  Phosphatic 
deposits  are  now  forming  in  the  ocean,  both  within  the  littoral 
belt  and  in  connection  with  the  globigerina-ooze,  etc.3 

Some  British  limestones4.  After  what  has  been  said 
in  the  foregoing  paragraphs,  a  few  remarks  on  some  of  the 
more  important  calcareous  formations  of  this  country  will  be 
sufficient  to  illustrate  our  subject. 

The  Cambrian  limestones  of  Burn  ess,  Assynt,  and  Skye 
are  remarkably  free  from  detrital  impurities.  They  are  in 
great  part  dolomitized,  presenting  a  saccharoid  texture. 

The  Bal$*Limestone  of  North  Wales  is  sometimes  a  fine 
calcareous  mud-stone,  sometimes  recrystallized.  The  most 
conspicuous  organic  fragments  are  those  of  crinoids,  which 
are  in  places  very  abundant,  and  polyzoa  are  also  found. 
The  Hirnant  Limestone5  has  a  peculiar  type  of  oolitic  struc- 
ture, the  grains  having  a  chalcedonic  skeleton  and  concentric 
zones  rendered  opaque  by  finely  divided  carbon.  The  Coniston 
Limestone  of  Westmorland  is  in  its  purer  parts  usually  recryst- 
allized throughout  to  a  granular  mass,  in  which  the  original 
characters  are  lost.  In  places  it  is  dolomitized6  (fig.  61).  In 
its  lower  part  it  contains  much  non-calcareous  material,  chiefly 
volcanic,  and  at  one  horizon  there  is  a  breccia  in  which  the 
enclosed  fragments  are  of  rhyolite,  andesite,  etc.,  the  matrix 
being  calcareous7.  At  Keisley,  in  the  Eden  Valley  district, 
the  rock  is  in  parts  coarsely  crystalline,  so  that,  while  the 

1  Chapman,  Q.  J.  G.  S.  (1892)  xlviii,  514-518,  pi.  xv  (Chalk,  Taplow). 

2  Strahan,  Q.  J.  G.  S.  (1891)  xlvii,  357-362  (Chalk,  Taplow)  ;  (1896) 
lii,  465  (Lewes). 

3  Murray  and  Renard,  '  Challenger1  Report,  Deep-Sea  Deposits  (1891), 
pi.  xx. 

4  Of  American  limestones  a  number  of  typical  examples  are  described 
by  Diller,  Educ.  Ser.  Rocks,  102-132. 

s  Fulcher,  G.  M.  1892,  114-117,  pi.  iv  ;  Harris,  Pr.  Geol.  Ass.  (1895) 
xiv,  78,  pi.  iv,  fig.  10. 

6  Q.  J.  G.  S.  (1893)  xlix,  367. 

^  Q.  J.  G.  S.  (1891)  xlvii,  309,  310. 


266 


SILURIAN   AND   DEVONIAN   LIMESTONES. 


outlines  of  the  larger  fossils  are  preserved,  all  minute  struct- 
ures are  destroyed.  At  a  lower  horizon  in  the  same  district  occur 
bands  composed  wholly  of  the  little  crustacean  Beyrichia. 

The  Wenlock  Limestone  of  Dudley,  with  a  recrystallized 
matrix,  still  preserves  abundant'  organic  fragments,  especially 
those  of  crinoids,  entomostracans,  trilobites,  corals,  polyzoans, 
and  brachiopods.  It  sometimes  has  as  much  as  30  per  cent. 
of  foreign  detrital  material.  At  Malvern  the  rock  is  largely 
oolitic,  the  grains  being  set  in  a  recrystallized  matrix,  and 
sometimes  themselves  recrystallized  (the  Wych).  Composite 
and  broken  oolitic  grains  also  occur  (Croft).  lAhe  Aymestry 
Limestone,  from  Dr  Sorby's  description1,  is  very  like  the 
Wenlock. 

Dr  Sorby2  has  pointed  out  many  interesting  features  in  the 
Devonian  limestones  of  Devonshire.  The  recognizable  organic 
fragments  are  chiefly  of  crinoids  and  corals,  and  the  finely 
divided  calcareous  matter  is  probably  derived  from  the  degrad- 
ation of  coral  skeletons.  This  fine  material  has  often  been 
recrystallized  in  the  usual  fashion,  the  impurities  being  segre- 
gated into  patches  of  finer  texture.  Again,  rhombohedral 
crystals  of  dolomite  (often  ferriferous)  have  frequently  been 
formed  in  the  rocks3,  and  some  have  become  true  dolomite -rocks, 
while  a  little  pyrites,  partly  oxidized,  is  not  uncommon.  Many 
of  the  rocks  shew  slaty  cleavage  in  every  respect  similar  to  that 
noticed  in  argillaceous  strata. 

The  Carboniferous  limestones  of  Clifton,  Bristol,  are  largely 
built  of  recognizable  organic  fragments.  Crinoids  and  some- 
times ostracods  are  especially  abundant  in  the  Lower  Lime- 
stones, foraminifera  and  the  problematical  organism  Calcisphera 
in  the  Middle4.  Numerous  oolitic  beds  occur,  and  in  some  of 

1  L.c.  p.  60. 

*.Phil.  Mag.  (1856)  ser.  4,  20-37.  See  also  Wethered,  Q.  J.  G.  S. 
(1892)  xlviii,  377-387,  pi.  ix. 

3  Wethered,  I.e.,  fig.  on  p.  381.     On  the  partial  silicification  of  some 
beds  see  Chapman,  G.  M.  1893,  100-104. 

4  Wethered,  G.  M.  1899,  78,  79,  and  Eep.  Brit.  Ass.  for  1898,  862, 
863  ;  see  also  G.  M.  1886,  529-540,  pi.  xiv,  xvi  (Forest  of  Dean),  and 
Morton's  Geol.  of  Liverpool  (2nd  ed.,  1891),  25-27  (Flintshire).     The 
microscopic  characters  of  some   Carboniferous  limestones  from  North 
Wales  and  from  Somerset  are  described  by  Beasley,  Pr.  Liverp.  G.  S. 
(1879)  iii,  359-361. 


BRITISH   CARBONIFEROUS   LIMESTONES.  267 

these  Mr  Wethered1  has  found  the  oolitic  structure  to  be 
connected  with  the  growth  of  Girvanella.  In  others  the 
oolitic  grains  are  in  some  measure  replaced  by  iron-oxides  and 
silica,  and  some  of  the  organic  fragments  (especially  of  polyzoa) 
also  shew  a  ferruginous  replacement  (fig.  58,  B).  The  inter- 
stitial calcareous  mud  is  usually  recrystallized  as  a  rather 
coarse  calcite-mosaic,  and  dolomitization  occurs  at  some 
horizons. 

The  Mountain  Limestone  of  the  North  of  England  is  on 
the  whole  of  similar  character.  The  most  frequent  of  the 
recognizable  organic  fragments  are  in  many  cases  those  of 
crinoids,  and  at  some  horizons  in  Derbyshire  and  Yorkshire 
these  constitute  the  main  bulk  of  the  rock,  but  fragments  of 
brachiopods,  corals,  polyzoa,  and  algae  also  occur,  and  may- 
be abundant,  while  foraminifera  are  often  very  plentiful2. 
Dr  Sorby  has  pointed  out  that  in  Derbyshire  some  of  the  beds 
are  pure  dolomite-rocks3. 

Dolomite-rocks  and  dolomitic  limestones  occur  at  many 
localities  in  the  Carboniferous  of  South  Wales4  and  of  Ireland5. 
They  are  in  general  highly  crystalline,  and  all  trace  of  organic 
structures  is  obliterated.  A  common  type  seems  to  be  that  in 
which  the  predominant  dolomite,  in  more  or  less  imperfect 
crystals,  is  cemented  by  calcite.  This  becomes  evident  on 
weathering,  when  the  removal  of  the  calcite  sets  the  dolomite 
crystals  free.  The  rocks  are  always  more  or  less  cellular  or 
porous,  but  the  cavities  are  commonly  filled,  or  lined  in  drusy 
fashion,  by  calcite.  Like  phenomena  occur  in  the  Carboni- 
ferous limestones  of  the  Isle  of  Man,  and  are  beautifully 
exhibited  on  the  shore  at  Castletown  and  Poolvash.  The 
resulting  dolomite-rock  is  often  quite  coarsely  crystalline. 

The  Permian  Magnesian  Limestone  is  in  general  a  true 
dolomite-rock,  and  in  most  cases  all  minute  original  structures 

1  Q.  J.  G.  S.  (1890)  xlvi,  270-274,  pi.  xi :  cf.  Harris,  Pr.  Geol.  Ass. 
(1895)  xiv,  76,  77,  pi.  iv,  figs.  7,  8. 

2  The  Saccamina  Limestone  of  Northumberland  is  an  example  of  a 
Carboniferous  rock  composed  essentially  of  foraminifera. 

3  See  also  Eutley,  Q.  J.  G.  S.  (1894)  1,  381,  382,  pi.  xix,  figs.  5,  6. 

4  Watts,  Mem.  Geol.  Sur.,  S.   Wales  Coalfield,  part  n  (1900),  34-36, 
pi.  i. 

5  Hardman,  Pr.  Eoy.  Ir.  Acad.  (1876)  ser.  2,  ii,  723-726. 


268 


PERMIAN   MAGNESIAN   LIMESTONE. 


have  been  lost  in  the  changes  which  converted  the  rock  to 
a  granular  mass  of  dolomite.  When  organic  fragments  are 
recognizable  they  are  most  frequently  those  of  shells  and 
polyzoa.  Locally  in  South  Yorkshire  the  latter  bodies  make 
up  almost  the  whole  of  the  rock  (Brodsworth,  Cadeby,  etc.). 
Near  Abergele  in  North  Wales  foraminifera  and  corals  form 
a  large  part.  Dr  Sorby  describes  the  Magnesian  Limestone 
north  of  Nottingham  as  comparatively  coarse-textured,  with 
evident  rhombohedral  crystals.  The  usual  type  in  Durham  is 
often  fine-grained,  the  elements  being  of  irregular  form.  Some- 
times an  interlocking  arrangement  of  the  granules,  aided  by 
the  presence  of  little  vacant  spaces-,  gives  a  certain  flexibility 
to  the  rock1  (Marsden).  The  little  cavities  or  pores  are, 
however,  as  in  other  dolomitic  rocks,  often  occupied  by  cryst- 
alline calcite.  The  well-known  nodules  of  Marsden  and 
Sunderland,  several  inches  in  diameter  and  with  well-marked 
radial  crystallization,  are  of  calcite  with  but  little  carbonate 
of  magnesia2.  The  Magnesian  Limestone  is,  as  a  rule,  tolerably 
free  from  foreign  detrital  matter,  but  locally  it  becomes  aren- 
aceous. Dolomitic  sandstones  occur  near  Mansfield,  and  the 
attenuated  representative  of  the  Magnesian  Limestone  in 
Westmorland  is  full  of  angular  quartz-grains. 

In  the  lower  Oolites  of  the  Cotteswold  and  Bath  districts3 
fragments  of  shells,  crinoids,  and  polyzoa,  tests  of  foraminifera 
and  other  organic  remains  are  recognized  in  variable  propor- 
tions. Most  of  these  limestones  are  oolitic,  but  the  original 
structure  of  the  oolitic  grains  is  often  destroyed  by  recryst- 
allization.  In  the  best  preserved  examples  Girvanella  is 
detected  at  various  horizons,  and  it  is  specially  well  exhibited 
in  the  coarse  pisolite  known  as  the  Tea  Grit4.'  The  rocks 
contain  various  small  proportions  of  insoluble  residue  consisting 
of  detrital  mineral  fragments  (quartz,  etc.). 

The  Lincolnshire  Limestone  and  Millepore  Oolite  of  the 

1  Card,  G.  M.  1892,  117-124. 

2  Garwood,  G.  M.  1891,  434-440. 

3  Wethered,  Q.  J.  G.  S.  (1890)  xlvi,  274-277,  pi.  xi ;    (1891)  xlvii, 
550-569,  pi.  xx ;  Pr.  Cottesiv.  F.  N.  Club  (1892),  x,  119,  120 ;  Harris,  Pr. 
Geol.  Ass.  (1895)  xiv,  70-72,  75,  76 ;  pi.  iv,  figs.  1,  2,  6. 

4  See  also  G.  M.  1889,  197,  198,  pi.  vi. 


BRITISH   JURASSIC    LIMESTONES.  269 

North  of  England1  are  made  up  largely  of  oolitic  grains  of  the 
ordinary  type,  consisting  of  a  nucleus  of  a  shell-fragment,  a 
quartz-grain,  or  a  brown  pellet  of  mud,  surrounded  by  numer- 
ous iron-stained  coats,  in  which  a  radial  structure  is  sometimes 
discernible.  The  organic  fragments  include  chips  of  brachio- 
pods  and  Pecten,  recrystallized  fragments  of  aragonite  shells, 
foraminifera,  valves  of  ostracods,  pieces  of  echinoderms,  etc., 
in  different  beds:  e.g.  abundant  brachiopod  spines  in  the 
Rhynchonella  spinosa  beds.  The  general  matrix  of  fine  calc- 
areous mud  is  almost  always  converted  into  a  crystalline 
calcite-mosaic  with  localisation  of  the  ferruginous  impurities, 
and  most  of  the  rocks  contain  a  considerable  amount  of  angular 
quartz-sand.  This  last  feature  is  more  prominent  in  the 
Scarborough  Limestone  and  the  Cornbrash.  The  former, 
especially  in  certain  nodular  bands,  is  often  an  iron-stone 
consisting  of  minute  rhombohedra  of  chalybite,  with  no  calcite 
remaining  except  in  the  fragments  of  shells. 

The  Coral  Oolite  of  Malton  is  another  good  specimen  of  an 
oolitic  limestone  with  recrystallized  matrix.  Besides  foramini- 
fera, crinoid  fragments,  etc.,  it  contains  abundant  remains  of 
aragonite  gasteropods  replaced  by  calcite  mosaic.  The  oolitic 
grains  are  sometimes  large  enough  to  be  termed  pisolitic,  but 
the  Girvanella  noticed  by  Mr  Wethered2  in  the  Osmington 
pisolite,  near  Weymouth,  is  not  yet  recorded  from  Yorkshire. 
The  last-named  author  (I.e.)  has  described  the  Portland  rocks3 
with  their  recrystallized  oolitic  grains.  The  silicification  of 
some  beds  in  that  district  has  already  been  referred  to. 

The  microscopic  characters  of  the  English  Chalk  have  been 
described  by  Dr  Sorby4,  Messrs  W.  Hill  and  Jukes-Browne6, 

1  Naturalist,  1890,  300-304.     For  figures  of  various  Lower   Oolitic 
limestones  see  Mem.  Geol.  Sur.,  Jurassic  Rocks,  vol.  iv,  pi.  i. 

2  G.  M.  1889,  197,  pi.  vi,  fig.  9  ;  Q.  J.  G.  S.  (1890)  xlvi,  277-279, 
pi.  xi,  figs.  6-8. 

3  On  the  Portland  Oolite  see  also  Harris,  Pr.  Geol.  Ass.  (1895)  xiv, 
72-74,  pi.  iv,  figs.  3,  4;  Teall,  Mem.   Geol.  Sur.,  Jurassic  Rocks,  vol.  v 
(1895),  186. 

4  Q.  J.  G.  S.  (1879)  xxxv,  Proc.  48,  49. 

5  Q.  J.  G.  S.  (1886-9)  xlii,  228-230,  Cambridge  and  Hertfordshire  ; 
242,  243,  Dover  ;  xliii,  580-585,  W.  Suffolk  and  Norfolk ;  xliv,  355-357, 
Lincolnshire  and  Yorkshire  ;  xlv,  406-413,  Berkshire  and  Wiltshire ;  see 


270 


CHALK. 


and  others.  The  tests  of  foraminifera,  and  especially  detached 
cells  of  Globigerina,  are  abundant  in  many  examples,  though 
they  rarely  form  the  chief  constituent  of  the  rock.  The  cells 
are  empty  in  the  soft  chalk  of  the  South,  but  filled  with  calcite 
in  the  hard  chalk  of  Yorkshire.  Radiolarian  remains  have 
been  preserved  only  exceptionally1.  Molluscan  fragments,  and 
especially  the  detached  shell-prisms  of  Inoceramus,  are  often 
well  represented :  in  the  Totternhoe  Stone  shell-fragments  form 
60  to  70  per  cent,  of  the  rock.  In  most  cases,  however,  the 
great  bulk  of  the  rock  consists  of  very  finely  divided  calcare- 
ous material,  the  nature  of  which  can  be  studied  only  by  rubbing 
the  chalk  with  water  and  examining  the  powder.  Coccoliths 
abound  in  this  fine  mud2,  but  the  minute  granules  are  mostly 
such  as  would  come  from  the  destruction  and  dissolution  of 
aragonite  shells,  corals,  etc.  Foreign  detrital  matter  is  rare  in 
the  Chalk,  except  at  certain  horizons,  but  is  abundant  in  the 
Red  Chalk  of  Hunstanton,  Lincolnshire,  and  Yorkshire3.  The 
Cambridge  Greensand  has  rather  large  quartz-grains,  with 
some  mica.  It  also  contains  a  considerable  number  of  glau- 
conite  grains,  usually  as  perfect  internal  casts  of  foraminifera4, 
and  glauconite  occurs  at  some  higher  horizons  in  smaller 
quantity.  Sponge-spicules  may  be  found  in  some  examples. 
Those  in  the  Lower  Chalk  of  Berkshire  and  Wiltshire  are 
sometimes  preserved  in  the  original  colloid  silica,  sometimes 
replaced  by  calcite,  while  little  globules  of  colloid  silica  ('0006 
inch  in  diameter)  occur  in  the  rock. 

also  Hume,  Chem.  and  Micro-miner.  Researches  on  the  Up.  Cret.  Zones 
of  the  S.  of  Engl.  1893,  and  on  the  Chalk  of  Antrim,  Q.  J.  G.  S.  (1897) 
liii,  568-584.  For  a  general  summary  of  the  microscopic  characters  of 
the  English  Chalk  see  Jukes-Browne,  Pr.  YorTcs.  Geol.  Pol.  Soc.  (1895) 
xii,  385-395. 

1  Hill  and  Jukes-Browne,  Q.  J.  G.  S.  (1895)  li,  600-603  (Melbourn 
rock). 

2  On  coccoliths  in  the  Chalk  see  Sorby,  Ann.  Mag.  Nat.  Hist.  (1861) 
ser.  3,  viii,  193-200. 

3  On  the  mineral  constitution   of  the  Eed  Chalk  and  its  insoluble 
residue  see  Mem.  Geol.  Sur.,  Cret.  Rocks,  vol.  i  (1900),  345,  346. 

4  Sollas,  Q.  J.  G.  S.  (1872)  xxviii,  399. 


CHAPTER  XIX. 

PYROCLASTIC   ROCKS. 

THE  fragmental  volcanic  rocks  are  in  general  the  products 
of  explosive  action1.  The  ejected  material  varies  from  the 
finest  dust  to  pieces  several  inches,  or  even  feet,  in  diameter, 
but  the  coarsest  types  do  not  require  special  notice  here. 

What  is  known  as  volcanic  dust  or  fine  ash  is  no  doubt 
partly  due  to  the  comminution  of  rocks  and  crystals  by  friction 
during  the  explosion,  but  a  great  part  of  it  must  represent 
lava  blown  out  from  the  vent  in  liquid  form  and  solidified 
almost  instantaneously  in  the  air.  It  doubtless  solidifies  as 
glass,  but  may,  of  course,  be  subsequently  devitrified.  The 
bodies  known  as  volcanic  bombs  and  lapilli  are  of  very  various 
sizes.  They  may  have  spheroidal  or  more  peculiar  forms ;  or 
again  they  may  be  irregularly  shaped  or  fitted  together.  Some 
kind  of  concentric  structure,  with  a  nucleus  and  an  outer  crust, 
is  often  seen,  or  the  exterior  may  be  scoriaceous.  In  many 
volcanic  accumulations  crystals  play  an  important  part.  They 
are  commonly  idiomorphic,  though  frequently  broken,  and 
belong  to  the  minerals  common  in  lavas.  They  may  sometimes 
be  torn  from  solid  rocks,  but  more  generally  they  must  have 
been  contained  in  a  fluid  matrix  before  the  eruption.  We  also 
find  rock  fragments,  either  angular  or,  in  submarine  deposits, 
partly  rolled  and  worn.  They  are  commonly  of  lava  for  the 

1  The  exceptions  ('flow-breccias,'  etc.)  are  not  important  for  our 
present  purpose. 


272  FRAGMENTAL   VOLCANIC   ROCKS. 

most  part,  shattered  and  blown  out  by  the  explosion,  but  we 
also  find  pieces  of  igneous  rocks  which  must  have  come  from 
greater  depths,  or  fragments  of  slate,  grit,  limestone,  etc.,  re- 
presenting strata  broken  through,  and  often  shewing  evident 
metamorphism.  The  larger  'ejected  blocks'  are  frequently 
of  these  foreign  and  non-volcanic  rocks. 

The  rocks  formed  by  the  accumulation  of  these  various 
materials  have  received  many  names.  The  term  ash,  applied 
to  the  finer  incoherent  products  of  modern  volcanoes,  is  some- 
times used  in  a  more  extended  sense;  but  the  older,  more  or 
less  compacted,  deposits  of  ash-material  are  usually  called  tuffs. 
A  large  proportion  of  them  were  evidently  laid  down  under 
water:  subaerial  accumulations  have  less  frequently  been 
preserved  from  destruction.  Rosenbusch,  in  describing  the 
ancient  acid  tuffs,  divides  them  into  compact  tuffs,  crystal- tuffs, 
and  agglomeratic  tuffs,  and  the  division  may  be  applied  to  rocks 
of  other  composition ;  but,  since  the  relative  proportions  of  dust, 
crystals,  and  lapilli,  etc.,  may  vary  to  any  extent,  no  precise 
divisional  lines  can  be  drawn.  If  angular  rock-fragments  be 
largely  represented,  the  deposit  is  termed  a  volcanic  breccia,  or 
if  the  fragments  be  rounded,  a  volcanic  conglomerate. 

According  to  the  nature  of  the  material,  the  rocks  may 
often  be  spoken  of  as  'rhyolite-tuff,'  'trachyte- tuff,'  etc.,  or, 
again,  'andesite-breccia,'  'trachyte-conglomerate,'  and  so  forth; 
but,  owing  to  the  admixture  of  various  materials,  the  rocks  do 
not  always  correspond  exactly  even  with  contemporaneous  lavas 
directly  associated  with  them. 

Further,  when  deposited  under  water,  the  volcanic  material 
may  become  mixed  with  ordinary  detritus  or  with  calcareous 
matter,  and  so  we  have  earthy  tuffs,  calcareous  tuffs,  etc.,  some 
of  which  are  fossiliferous. 

General  characters.  Fragmental  volcanic  rocks  have 
received  much  less  minute  study  than  lavas,  and  indeed  present 
greater  difficulty,  requiring  for  the  finer  material  the  use  of 
high  magnifying  powers. 

Typical  volcanic  dust  in  a  fresh  state  seems  to  consist 
essentially  of  glass-particles,  with  only  a  minor  proportion  of 
comminuted  crystals  and  microlites.  The  glass-fragments 


VOLCANIC  DUST.  273 

have  a  peculiar  structure  and  a  characteristic  form.  This  is 
due  to  the  immense  number  of  contained  steam -bubbles,  which 
are  drawn  out  into  minute  tubes,  causing  the  glass  to  break 
into  linear  shapes  with  a  longitudinal  striation.  The  glass 
is  distinguished  from  comminuted  felspar  by  the  absence  of 
true  rectilineal  boundaries  and  the  isotropic  character.  The 
minute  fragments  are  colourless,  except  in  the  case  of  basic 
glasses,  which  may  be  of  a  brown  tint.  According  to  Murray 
and  Renard1,  the  characteristic  appearance  of  these  glass 
fragments  may  be  recognized  even  in  excessively  small  particles 
(less  than  '0002  inch),  while  the  distinctive  properties  of  most 
minerals  cannot  be  detected  in  fragments  of  smaller  dimensions 
than  '002  inch.  The  minerals  commonly  recognized  are  the 
familiar  constituents  of  volcanic  rocks — especially  plagioclase, 
pyroxenes,  and  magnetite2,  for  many  of  these  very  fine  volcanic 
dusts  are  of  the  nature  of  pyroxene-andesite.  The  crystals  are 
often  coated  with  glass  or  have  glass  adherent. 


FIG.  63.    BASIC  TUFF,  ORDOVICIAN,  WET  SLEDDALE  NEAR  SHAP  ;     x  20. 

The  bulk  of  the  rock  is  of  very  fine  particles,  but  encloses  some  rock- 
fragments  and  numerous  crystals  of  felspar,  which  tend  to  stand 
perpendicularly  to  the  lamination  of  the  matrix  [895]. 

1  See  especially  Nature  (1884),  xxix,  585-589. 

2  Fouque  and  Michel  Le~vy,  pi.  xm,  fig.  4. 

H.  p.  18 


274  CHARACTERS   OF   VOLCANIC   TUFFS. 

The  authors  named  find  precisely  similar  material  to  be 
widely  distributed  in  modern  deep-sea  deposits,  where  it 
accumulates  from  the  fall  of  wind-borne  dust  and  the  disin- 
tegration of  floating  pumice. 

In  tuffs  formed  not  far  from  a  volcanic  centre,  crystals  of 
recognizable  size,  perfect  or  broken,  are  often  embedded  in  a 
fine-textured  matrix.  These"  frequently  shew  a  characteristic 
arrangement,  standing  with  their  long  axes  vertical  or  roughly 
perpendicular  to  the  lamination  of  the  matrix,  as  if  dropped 
into  their  place  from  above  (fig.  63). 

In  any  except  comparatively  young  tuffs  the  original 
character  of  the  finely  divided  material  is  largely  obscured  by 
secondary  changes,  the  loose  texture  of  the  deposits  rendering 
them  peculiarly  liable  to  alteration.  According  to  the  nature 
of  the  rock,  such  minerals  as  quartz,  sericitic  mica,  chlorite, 
calcite,  etc.,  are  developed  at  the  expense  of  the  original  dust. 
Silicification  is  very  common  in  the  acid  tuffs.  Fragments  of 
lava  naturally  suffer  less  than  the  enclosing  matrix,  but  if 
glassy  they  readily  become  altered.  In  particular  the  more 
basic  glasses,  such  as  basalt  and  augite-andesite,  are  hydrated 
and  converted  into  the  transparent  brown  or  yellow,  or  more 
rarely  green,  substance  known  as  palagonite1. 

In  some  cases  it  is  very  difficult  to  distinguish  compact 
rhyolite-tuffs,  silicified  or  otherwise  altered,  from  rhyolites 
which  have  undergone  similar  changes,  the  lamination  of  the 
one  and  the  flow-structure  of  the  other  often  increasing  the 
resemblance.  When  enclosed  crystals  occur,  their  character- 
istic orientation,  as  noted  above,  will  often  furnish  a  clue ;  or 
again  the  occurrence  of  fragments  with  concave  outlines2  (Ger. 
Bogenstructur)  is  sufficiently  suggestive  (fig.  64,  A).  Old  tuffs 
of  andesitic  or  basaltic  composition,  when  more  or  less  cleaved 
and  impregnated  with  secondary  chlorite,  calcite,  and  other 
substances,  may  sometimes  be  mistaken  for  crushed  lavas  of 
like  composition,  or  vice  versa,  unless  distinct  fragments,  such 

1  For  some  discussion  of  the  nature  of  this  substance,   see  Zirkel, 
Micro.   Petr.   Fortieth  Parallel,   pp.    273-275  (1876).     The  basic  glass 
which  has   not   suffered  hydration  is  sometimes  termed  sideromelane : 
see  also  Murray  and  Renard,   Ghall.  Rep.,  Deep-Sea  Deposits  (1891). 

2  Rutley,  Q.  J.  G.  S.  (1902)  Iviii,  p.  30,  fig.  (Builth). 


RHYOLITIC  TUFFS.  275 

as  lapilli,  can  be  detected.  These  lapilli  can  often  be  recog- 
nized by  a  rounded  outline,  or  a  vesicular  structure,  or  an 
opacity  due  to  finely  divided  magnetite1. 

It  will  easily  be  understood  that  fine-textured  tuffs  may 
exhibit  precisely  the  same  phenomena  of  slaty  cleavage  as 
those  seen  in  argillaceous  sediments,  while  the  coarser  pyro- 
clastic  rocks  (volcanic  breccias  and  agglomerates)  are  more 
readily  crushed  than  solid  rocks  such  as  lavas. 

Illustrative  examples.  Without  attempting  to  deal 
systematically  with  the  great  variety  of  tuffs,  agglomerates, 
etc.,  it  will  be  sufficient  to  draw  attention  to  a  few,  which 
have  been  already  described,  and  illustrate  various  points  of 
interest.  As  typical  of  many  fine  volcanic  dusts,  we  take  that 
which  was  spread  over  a  vast  extent  of  country  after  the  great 
eruption  of  Krakatau  in  1883.  This  has  been  described  by 
several  writers2.  About  nine- tenths  of  the  material  consists 
of  glass  fragments  with  the  characteristic  features  noticed 
above.  The  remainder  is  of  comminuted  crystals  of  plagio- 
clase,  magnetite,  enstatite,  and  augite,  the  whole  having  the 
composition  of  an  acid  pyroxene-andesite. 

We  pass  on  to  notice  a  few  consolidated  deposits  (tuff's)  of 
various  composition. 

An  interesting  study  of  ancient  acid  tuffs  has  been  made 
by  Miigge  in  the  Devonian  of  the  Lenne  district  in  Westphalia. 
The  rocks  are  associated  with  old  soda-rhyolites  ( 'Keratophyre' 
of  the  author),  and  have  a  similar  composition.  They  are  for 
the  most  part  of  compact  type,  and,  though  considerably 
altered,  still  retain  much  that  was  characteristic  in  their 
original  structure.  In  particular,  they  often  shew  very  clearly 
the  peculiar  form  of  the  constituent  ash-particles,  bounded  by- 
concave  curves,  which  clearly  suggest  broken  up  pumice3. 
Crystal-tuff's  are  also  found. 

1  Cf.,  e.g.,  Teall's  figure  of  one  of  the  Llanberis  tuffs,  pi.  XLV,  fig.  1. 

2  Murray  and  Eenard,  Nature  (1884)  xxix,  585-589 ;  Cole,  Proc.  Geol. 
Ass.  (1884)  viii,  332-335 ;  Joly,  Proc.  Roy.  Dubl.  Soc.  (1884)  N.  S.  iv, 
291-299,  pi.  xii,  xin ;  Judd,  Eep.  Krak.  Comm.  Roy.  Soc.  (1888)  38-41, 
pi.  rv. 

3  Neu.  Jahrb.  fur  Min.,  Beil.  Bd.  viii,  pi.  xxiv,  figs.  20,  21,  etc.  (1893). 
All  the  figures  illustrating  this  paper  are  instructive.     See  also  chromo- 
lithograph in  Benverth,  Lief.  in. 

18—2 


276  EHYOLITIC   TUFFS   OF  WALES. 

Some  of  the  Ordovician  rhyolite- tuffs  of  Caernarvonshire 
have  much  resemblance  to  those  just  mentioned1.  Others, 
there  and  in  the  Lake  District,  have  evidently  consisted  of 
much  more  finely  divided  material,  and  have  often  lost  all 
trace  of  their  original  characteristics  by  secondary  changes. 
Embedded  crystals  usually  occur  (Glyder  Fawr,  etc.),  but  do 
not  make  up  any  large  part  of  the  mass.  There  are,  however, 
beds  made  up  very  largely  of  small  rock-fragments  and  broken 
crystals  lying  in  a  fine-textured  matrix  or  united  by  a  brown 
ferruginous  paste.  The  rock-fragments  are  of  various  quartz- 
porphyries  and  granophyres,  and  sometimes  detached  spherules ; 
the  crystals  are  of  acid  felspar  and  decomposed  augite  (near 
Llanbedrog,  etc.)2.  Prof.  Bonney3  has  described  an  agglpmer- 
atic  type  from  the  older  rocks  of  the  Llanberis  district  as 
consisting  of  fragments  and  lapilli  of  rhyolite  and  fragments 
of  quartz  and  felspar  embedded  in  an  altered  felspathic  dust. 
Here  some  of  the  rock-fragments  are  of  large  size.  Concerning 
the  tuffs  of  the  Arenig  district  there  is  little  information, 
but  examples  have  been  described  by  Mr  Cope4  from  Aran 
Mowddwy. 

Rhyolite-tuffs  occur  at  several  places  in  Pembrokeshire. 
One  from  Llanrian  shews  a  very  characteristic  micro-structure 
(fig.  64,  A).  Others  have  been  described  from  Fishguard5 
and  from  St  David's6.  Good  rhyolite-tuffs  are  found  in  the 
Malvern  district  (Knighton). 

Some  of  the  fine-textured  rocks  which  have  been  styled 
'porcellanite'  and  'halleflinta'  are  acid  tuffs  compacted  by 
secondary  silica  and  other  substances.  Examples  occur  in  the 
St  David's  district  (Clegyr  Bridge,  etc.)  and  in  Cham  wood 
Forest  (Nanpanton).  Rocks  of  the  same  general  aspect  in  the 
Lake  District  (Bow  Fell,  etc.)  are  fine  tuffs  of  intermediate 
composition. 

A  number  of  rhyolitic  tuffs  and  breccias  are  described  by 

1  Miigge,  I.e.,  pi.  xxvii,  fig.  41. 

2  Bala  Vole.  Ser.  Gaern.,  27. 

3  Q.  J.  G.  S.  (1879)  xxxv,  312. 

4  Pr.  Liverp.  G.  S.  (1897)  viii,  pi.  iv,  v. 
6  Eeed,  Q.  J.  G.  S.  (189$)  li,  175. 

6  Geikie,  ibid.  (1883)  xxxix,  297-301. 


TRACHYTIC   TUFFS. 


277 


ZirkeP  from  the  Tertiary  volcanic  region  of  Nevada,  while 
others  occur  among  the  ancient  volcanic  rocks  of  the  eastern 
States2. 


B 


Fio.  64.    ANCIENT  PUMICEOUS  TUFFS;     x20. 

A.  Ehyolitic   Tuff   (Ordovician),  Llanrian,  Pembrokeshire.     Origi- 
nally composed  wholly  of  glass-fragments,  the  outlines  of  which  are  still 
partly  discernible,  despite  their  alteration  [475]. 

B.  Andesitic   Tuff  (Old   Red   Sandstone),   Inverinan,   Argyllshire. 
The  outlines  of  many  of  the  fragments  are  clearly  indicated,  owing  to 
their  being  charged  with  magnetite-dust.     There  are  also  partly  rounded 
crystals  of  plagioclase  and  an  occasional  flake  of  biotite  [2385]. 

Trachyte-tuffs  of  various  types  are  known  in  many  of  the 
newer  volcanic  districts  of  Europe  and  America,  but  they  have 
not  often  been  minutely  studied.  The  rock  known  as  'trass' 
is,  at  least  in  part,  of  this  nature.  In  the  Siebengebirge  is 
a  considerable  development  of  trachyte-conglomerate.  The 
leucitophyres  of  the  Eifel  district  are  accompanied  by  tuffs  of 
corresponding  nature. 

A  good  example  of  a  hornblendic  andesite-tuff  is  extensively 
developed  at  Rhobell  Fawr3  in  Merioneth,  an  old  volcano 

1  Micro.  Petrogr.  Fortieth  Parallel  (1876),  264-271. 

2  See,  e.g.,  Or.  0.  Smith,  Geology  of  Fox  Island,  Me.  (1896)  39  and 
pi.  i,  fig.  4. 

3  Cole,  G.  M.  1893,  343. 


278  ANDESITIC   TUFFS. 

probably  of  late  Cambrian  age.  It  is  in  great  measure  a 
crystal-tuff,  the  most  conspicuous  elements  being  perfect  and 
broken  crystals  of  brown  hornblende  and  pale  yellowish  augite. 
Similar  rocks,  of  Ordovician  age,  are  found  at  Bail  Hill  in 
Ayrshire1.  Tuffs  with  the  general  composition  of  hornblende- 
and  especially  of  mica-andesites,  enclosing  broken  crystals 
and  lapilli,  occur  in  the  Old  Red  Sandstone  of  the  Oban 
district2  (fig.  64,  B). 

The  majority  of  the  Ordovician  tuffs  in  the  Lake  District 
correspond  in  general  composition  with  andesites  and  with 
basic  andesites  or  basalts,  but  many  of  them  have  in  addition 
angular  fragments  of  rhyolite.  Crystals  of  felspar  are  often 
seen,  but  do  not  make  up  a  large  part  of  the  rocks,  which  are 
essentially  of  the  compact  type  in  most  cases  (fig.  63).  Rolled 
pieces  of  lava  of  small  dimensions  may  occur.  In  some  localities 
the  rocks  consist  mainly  of  a  mixture  of  small  lapilli  with 
fragments  of  slate,  grit,  etc.,  often  metamorphosed.  Mr 
Hutchings  has  described  an  example  from  Falcon  Crag  near 
Keswick3.  The  finer  tuffs  of  the  district  are  often  cleaved 
and  highly  altered  (see  below). 

The  cleaved  tuffs  of  Cader  Idris4  in  Merioneth  also  contain 
plenty  of  slate-fragments  with  felspar  crystals  and  particles  of 
scoriaceous  andesite-glass  converted  into  green  palagonite,  all 
set  in  a  fine  ashy  matrix. 

Other  Ordovician  tuffs  consist  largely  of  little  fragments 
of  formerly  glassy  and  sometimes  pumiceous  andesite,  now 
converted  into  a  palagonite-like  material  of  yellow  or  brown 
colour  (e.g.  Snead  in  Shropshire)5.  Tuffs  mainly  of  andesitic 
material  are  found  also  in  the  Bala  series  of  Lambay  Island, 
near  Dublin6. 

Some  interesting  fragmental  rocks  of  basic  composition 
occur  in  the  old  volcanic  series  of  St  David's  of  early  Cambrian 

1  Teall,  Ann.  Rep.  Geol.  Sur.  for  1896,  39. 

2  Kynaston,  ibid.,  54,  and  Tr.  Edin.  G.  S.  (1901)  viii,  87-90,  pi.  m. 

3  G.  M.  1891,  462. 

4  Cole  and  Jennings,  Q.  J.  G.  S.  (1889)  xlv,  423-431. 

5  Cole,  Q.  J.  G.  S.  (1888)  xliv,  pi.  xi,  fig.  5. 

6  Gardiner  and  Keynolds,  Q.  J.   G.  S.  (1898)  liv,  140,  141 ;  Sollas, 
Pr.  Geol.  Ass.  (1893)  101,  with  figs.  7,  8. 


BASALTIC   TUFFS.  279 

or  pre-Cambrian  age.  They  are  agglomeratic  tuffs  consisting 
chiefly  of  little  fragments  of  basic  lava,  sometimes  rounded 
but  usually  angular  or  subangular.  In  some  there  is  very 
little  matrix :  it  consists  of  fine  debris  of  the  same  material  as 
the  larger  fragments.  Sir  A.  Geikie1  has  described  specimens 
from  Pen-y-foel  and  Pen-maen-melyn. 

Among  the  basaltic  rocks  crystal- tuffs  seem  to  be  almost 
unrepresented.  A  common  type  consists  of  lapilli  of  basalt 
(glassy  or  altered)  cemented  by  calcite,  aragonite,  limonite, 
etc.  Widely  distributed  is  the  palagonite  type2  of  Walters- 
hausen,  described  from  Sicily,  Iceland,  the  islands  of  the 
Pacific,  etc.  This  consists  chiefly  of  little  fragments  of  altered 
glassy  basalt,  usually  of  brown  colour,  often  vesicular,  and 
sometimes  enclosing  a  few  crystals  of  augite,  olivine,  or  basic 
plagioclase ;  while  the  cementing  material  is  obtained  from  the 
decomposition  of  the  fragments,  or  may  include  calcite  derived 
from  calcareous  matter  contemporaneously  deposited  or  by 
infiltration  from  without.  Such  rocks  are  widely  represented 
among  the  older  formations  in  this  country.  Palagonite-tuffs, 
as  well  as  other  basalt-tuffs,  occur  in  Nevada,  etc.3 

Submarine  tuffs  of  intermediate  and  basic  composition 
occur,  for  example,  abundantly  in  the  Carboniferous  in  the 
basin  of  the  Firth  of  Forth.  Most  of  them  contain  some 
admixture  of  detrital  or  calcareous  matter,  but  characteristic 
examples  of  tuffs,  and  in  particular  of  palagonite-tuffs,  are 
found.  As  described  by  Sir  A.  Geikie4,  the  bedded  de- 
posits consist  of  a  fine-textured  matrix  enclosing  fragments 
of  lava.  The  latter  are  the  debris  of  already  consolidated 
rocks  rather  than  typical  lapilli:  they  are  largely  vesicular, 
not  only  at  the  margin  but  throughout,  and  the  vesicles  are 
often  cut  by  the  external  surface  of  the  fragment.  Calcite, 
delessite,  etc.,  occupy  the  cavities.  A  common  feature  is 
fragments  of  a  transparent  green  or  yellowish  material  re- 

1  Q.  J.  G.  8.  (1883)  xxxix,  295-300,  pi.  ix,  figs.  1,  2. 

2  For  figures   of  palagonite-tuffs  see  Zirkel,  Micro.   Petr.  Fortieth 
Parallel,  pi.  xn,  figs.  3,  4;   Kosenbusch,  Mass.  Gest.,  pi.  vi,  fig.  4. 

3  Zirkel,  Micro.  Petr.  Fortieth  Parallel  (1876),  272-275. 

4  Trans.  Roy.  Soc.  Edin.  (1879)  xxix,  513-516,  pi.  xn,  fig.  10.    For 
examples  of  similar  rocks  of  pre-Cambrian  age,  see  E.  D.  Irving,  Copper- 
bearing  Rocks  of  L.  Superior  (1884),  pi.  xv. 


280  BASALTIC  TUFFS. 

sembling  serpentine,  which  is  evidently  an  altered  vesicular 
glass,  and  is  referred  to  palagonite.  The  matrix  of  these 
rocks  has  probably  consisted  of  finely  divided  material  of  the 
same  general  nature  as  the  larger  fragments,  but  its  structure 
is  completely  obscured  by  secondary  changes,  and  the  mass  is 
stained  green  or  brown.  Tuffs  of  essentially  similar  characters 
are  found  in  the  Carboniferous  of  the  Isle  of  Man  (Scarlet 
Point)1  and  the  Limerick  district2. 

The  tuffs  associated  with  the  Carboniferous  of  Derbyshire 
are  in  great  part  composed  of  true  lapilli,  often  bordered,  and 
having  numerous  vesicles  not  broken  by  the  outline  of  the 
lapillus3.  The  material  is  a  brown  glass  with  globulites  and 
crystallites  and  with  crystals  of  olivine  or  plagioclase.  These 
minerals  are  often  replaced  by  calcite,  and  the  same  substance 
fills  the  vesicles  and  forms  the  cement  of  the  rock. 

Many  basic  tuffs  have  a  calcareous  cement.  In  some  cases 
the  calcite  may  have  been  derived  from  the  destruction  of  lime- 
bearing  silicates  or  introduced  in  solution  from  an  extraneous 
source.  There  are,  however,  many  submarine  tuffs  of  all  ages 
in  which  calcareous  organisms  have  been  accumulated  contem- 
poraneously with  the  volcanic  material,  giving  rise  to  every 
gradation  from  a  pure  tuff  to  a  pure  limestone.  Such  deposits 
are  forming  at  the  present  day  in  the  neighbourhood  of  volcanic 
islands,  and  consolidated  calcareous  tuffs,  often  abounding  in 
foraminifera,  etc.,  are  beautifully  represented  among  the  Recent 
strata  of  the  Solomon  Islands4,  the  Tonga  group5  (fig.  65), 
Torres  Straits6,  etc. 

Fine-grained  tuffs,  and  in  a  less  degree  agglomerates,  may 
receive,  as  already  mentioned,  a  secondary  cleavage-structure 
precisely  similar  to  that  observed  in  argillaceous  rocks;  and 
the  cleavage  is  often  accompanied  by  mineralogical  changes. 

1  Hobson,  Q.  J.  G.  S.  (1891)  xlvii,  442,  443. 

2  Watts,  Guide,  95. 

3  Arnold-Bemrose,  Q.  J.  G.  S.  (1894)  1,  625-642,  pi.  xxiv,  figs.  4,  5, 

XXV. 

4  Guppy,  Trans.  Roy.  Soc.  Edin.  (1885)  pi.  CXLV,  etc. 
6  G.  M.  1891,  251-256. 

6  Sollas  and  Cole,  Sci.  Proc.  Roy.  Dubl.  Soc.  (1891)  vii,  124-126; 
Haddon,  Sollas,  and  Cole,  Trans.  Roy.  Ir.  Acad.  (1894)  xxx,  436,  437, 
pi.  xxv,  fig.  6. 


CLEAVED   TUFFS   OF   LAKE   DISTRICT. 


281 


The  cleaved  tuffs  or  ash-slates  of  the  Lake  District  have  been 
noticed  by  Dr  Sorby,  and  some  of  them  described  in  detail  by 
Mr  Hutchings1.  These  rocks  are  of  intermediate,  and  some- 
times perhaps  basic,  composition,  and  the  finely  divided 
portions  have  undergone  great  secondary  changes.  Chlorite 
and  dust  or  granules  of  calcite  are  often  conspicuous,  and 


FIG.  65.     CALCAREOUS  TUFF,  EUA,  TONGA    SLANDS  ;     x20. 

The  fragments  are  mainly  of  brown-stained  andesitic  and  basic  lava, 
more  or  less  glassy  and  altered  to  palagonite.  These,  with  tests  of 
foraminifera  (/o),  are  enclosed  in  a  calcareous  matrix.  Each  foram- 
iniferal  chamber  is  occupied  by  calcite  with  radial  fibrous  structure, 
giving  a  perfect  black  cross  between  crossed  nicols,  and  the  same  is  seen 
in  the  little  spherical  bodies  (s),  which  are  doubtless  detached  chambers 
of  Globigerina  [1273]. 

when  these  have  been  removed  by  acid  from  the  powdered 
rock,  or  from  very  thin  slices,  other  minerals  may  be  detected, 
especially  minute  sericitic  mica,  which  gives  bright  polarization- 
tints.  The  needles  of  rutile,  so  characteristic  of  clay-slates, 
are  not  found,  but  there  are  sometimes  granules  of  sphene 
(e.g.  Kentmere).  In  some  of  these  slates  minute  garnets  play 
an  important  part  (e.g.  Mosedale,  near  Shap).  In  general 

1  G.  M.  1892,  154-161,  218-228 ;  see  also  Pr.  Liverp.  G.  S.  (1901) 
ix,  106-112,  pi.  vi,  vn. 


282  CLEAVED  TUFFS   OF   LAKE   DISTRICT. 

there  has  been  an  abundant  separation  of  silica,  partly  as 
quartz,  partly  perhaps  as  chalcedony. 

This  is  the  general  character  of  the  finest  slates  of  the 
Lake  District,  which  are  evidently  greatly  altered  from  their 
original  state.  The  coarser  bands  have  a  matrix  of  similar 
character  enclosing  lapilli  and  recognizable  fragments  of 
andesite  and  also  of  rhyolite.  Some  rocks  of  a  comparatively 
coarse  agglomeratic  nature  are  worked  for  slates  in  Borrow- 
dale. 


APPENDIX  TO  SEDIMENTARY  ROCKS. 


A  FEW  bedded  rocks,  not  included  in  the'foregoing  chapters, 
deserve  brief  notice.  They  are  deposits  due,  some  to  chemical, 
others  to  organic  agency.  We  shall  exclude  the  carbonaceous 
rocks  (coal,  etc.),  which  belong  to  the  domain  rather  of  fossil 
botany  than  of  petrology. 

There  are  certain  salts  which  occur  in  beds,  forming  strati- 
fied rocks,  and  locally  attain  a  great  development.  One  of  these 
is  rock-salt,  found  in  the  Trias  of  Cheshire  and  Worcestershire 
and  at  various  geological  horizons  in  other  countries.  Besides 
admixture  of  other  salts,  the  deposits  contain  more  or  less  of 
clayey,  organic,  or  other  impurities.  Rock-salt  itself  (sodium 
chloride)  is  colourless  in  slices,  and  has  a  strongly  marked 
cubic  cleavage  and  a  low  refractive  index.  It  frequently  con- 
tains microscopic  brine-cavities  of  cubical  shape. 

Another  mineral  which  may  form  a  rock  by  itself  is 
gypsum1.  It  occurs  in  allotriomorphic  grains  which  may  be 
very  small.  The  strong  clinopinacoidal  cleavage  is  well  marked ; 
the  refractive  index  is  quite  low,  and  the  double  refraction 
is  weak  (about  equal  to  that  of  quartz).  Gypsum  is  often 
associated  with  rock-salt. 

The  simple  sulphate  of  lime  anhydrite  is  also  found  as  a 
rock  (Val  Canaria  in  Switzerland,  etc.),  building  allotriomorphic 
to  partly  idiomorphic  crystals  or  fibrous  aggregates.  The  two 
strong  pinacoidal  cleavages  are  well  marked;  the  refractive 

1  Hammerschmidt  notes  that  the  heating  of  the  Canada  balsam  in 
mounting  may  cause  partial  dehydration  of  the  mineral,  giving  rise  to 
little  matted  aggregates  of  anhydrite.  On  gypsum  in  general  see  Good- 
child,  Proc.  Geol.  Ass.  (1888)  x,  425-445. 


284  CLAY-IRONSTONES  :    CHERTS. 

index  is  low,  but  the  double  refraction  very  strong  (equal  to 
that  of  muscovite).  The  rock  often  encloses  grains  of  rock-salt 
or  of  dolomite  and  other  minerals.  It  is  specially  liable  to 
conversion  into  gypsum,  which  may  be  seen  in  various  stages, 
veins  and  patches  of  the  latter  mineral  traversing  the  anhydrite 
mass.  This  involves  an  increase  of  bulk  and  phenomena  of 
disruption. 

Another  mineral  which  sometimes  forms  a  simple  rock  is 
chalybite  or  siderite,  the  ferrous  carbonate.  We  have  already 
seen  that  some  iron-stones  of  this  composition  have  been  formed 
by  metasomatic  processes  from  limestones,  but  in  other  cases, 
such  as  the  ironstone  bands  in  the  Coal-measures  of  Yorkshire, 
etc.,  there  is  no  evidence  of  such  an  origin.  The  mineral  may 
be  mixed  with  other  carbonates  in  smaller  proportions  and 
with  a  variable  quantity  of  argillaceous  matter  (clay-ironstone). 
Chalybite  has  the  rhombohedral  cleavage  of  the  calcite-group 
of  carbonates,  and  in  its  brownish-yellow  colour  resembles  some 
impure  dolomites. 

Some  siliceous  rocks.  Here  we  may  notice  certain 
siliceous  rocks  which  do  not,  at  least  in  the  main,  result  from 
pseudomorphism  of  limestones.  Some  well-known  examples  of 
cherts  fall  under  this  head,  the  silica  being  derived  from  siliceous 
sponges,  recognizable  remains  of  which  still  form  an  important 
part  of  the  rock.  In  the  chert-beds  of  the  Upper  Greensand1 
in  the  Isle  of  Wight  the  sponge-spicules  sometimes  remain  in 
their  original  condition,  consisting  of  colloid  (isotropic)  silica, 
but  more  usually  spicules  and  matrix  are  alike  converted  to 
chalcedony.  The  sponge-beds  of  similar  age  in  the  Weald 
district  consist  largely  of  colloid  silica,  but  the  spicules  are 
represented  by  empty  casts.  The  cherts  of  the  Carboniferous 
limestones  of  Ireland2  are  found,  in  the  best-preserved  speci- 
mens, to  consist  largely  of  sponge-spicules,  the  matrix  being 
also  siliceous  and  doubtless  derived  from  the  dissolution  of 
other  spicules.  Here  the  silica  is  always  in  the  condition  of 
chalcedony  or  quartz.  The  Yoredale  cherts  of  Yorkshire  and 


1  Hinde,  Phil.   Trans.  Roy.    Soc.   (1885),  pp.  447,  448,  pi.  40;   for 
abstract  see  Q.  J.  G.  S.  (1889)  xlv,  406,  407. 

2  Hinde,  G.  M.  1887,  441-443. 


RADIOLARIAN   ROCKS.  285 

North- Wales  are  of  similar  character,  with  better  preserved 
sponge-remains,  and  the  same  seems  to  be  true  of  the  Car- 
boniferous cherts  (Fr.  phthanites)  of  Belgium,  and  of  some 
other  countries,  including  some  of  the  'Kieselschiefer'  of  the 
Germans. 

Of  great  interest  are  the  deep-sea  deposits  which  give  rise 
to  siliceous  rocks.  The  'Challenger'  Expedition1  has  shewn 
that  these  occur  over  extensive  tracts  of  the  ocean-floor  in  its 
deepest  portions.  Characteristic  types  are  the  diatom-ooze, 
essentially  an  accumulation  of  the  frustules  of  diatoms2,  and 
the  radiolarian  ooze,  made  up  mainly  of  the  tests  of  radiolaria. 
There  may  be  some  admixture  of  finely  divided  volcanic  ma- 
terial or  decomposition-products  or  of  foramini feral  remains. 
The  equivalents  of  this  radiolarian  ooze  are  found  in  Recent  and 
Tertiary  radiolarian  earths  such  as  those  of  Barbados3  and 
Trinidad4,  and,  in  a  compacted  form,  in  the  radiolarian  cherts 
of  some  of  the  older  formations.  The  Ordovician  cherts  of  the 
south  of  Scotland,  described  by  Dr  Hinde5,  shew  in  slices  a 
faint  cloudy  appearance,  giving  a  mottled  effect  between  crossed 
nicols,  but  are  frequently  veined  and  stained  with  dark  brown. 
In  the  transparent  parts  the  radiolaria  shew  as  shadowy  circles 
denned  by  their  interior  being  somewhat  lighter  than  the  sur- 
rounding matrix.  In  the  stained  parts  the  tests  are  replaced 

1  See  especially  Murray  and  Renard,  Chall.  Rep.,  Deep- Sea  Deposits 
(1891)  with  plates  (pi.  xv,  etc.). 

2  Diatomaceous  deposits  of  more  limited  extent  are  found  occupying 
the  sites  of  old  lakes  at  numerous  places  in  the  north  and  west  of  Scot- 
land ;  Macadam,  M.  M.  (1884)  vi,  87-89,  etc. :  also  at  Toome  Bridge  on 
Loch  Neagh;  Pollok,  Sci.  Pr.  Roy,  Dubl.  Soc.  (1899)  ix,  33-36,  with 
figures.     Similar  deposits  occur  near  Monterey,  Cal.  and  White  Plains, 
Nev. ;  Diller,  136,  137 :  also  in  the  Tertiary  of  Richmond,  Va.     These 
pulverulent  deposits  of  diatoms  are  styled  '  Kieselguhr,'  '  tripolite,'  and 
'  diatomite.' 

3  See  Jukes-Browne  and  Harrison,  Q.  J.  G.  S.  (1892)  xlviii,  174,  175 ; 
Nicholson  and  Lydekker,  p.  34,  fig.  12. 

4  Gregory,  Q.  J".  G.  S.  (1892)  xlviii,  538,  539.    On  a  radiolarian  earth 
from  S.  Australia  see  Hinde,  Q.  J.  G.  S.  (1893)  xlix,  221,  pi.  v. 

5  Ann.  Mag.  Nat.  Hist.  (1890)  ser.  6,  vi,  41-47,  pi.  in,  iv.     On  a 
somewhat    similar    rock  from   Mullion  Island,    Cornwall,   see   Hinde, 
Q.  J.  G.  S.  (1893)  xlix,  215,  pi.  iv ;  on  radiolarian  cherts  in  the  Culm 
of  Devon,  Cornwall,  and  Somerset,  see  Hinde  and  Fox,  0.  J.  G.  S.  (1895) 
li,  629-634. 


286  SILICEOUS  SINTER. 

by  a  dark  substance,  and  may  retain  much  of  their  original 
structure.  The  most  considerable  development  of  radiolarian 
cherts  known  is  in  the  Devonian  of  Tamworth,  N.  S.  W4.  Here 
a  deep-sea  origin  is  inadmissible. 

Radiolarian  cherts  and  jaspers  of  Mesozoic  age  are  known 
from  several  localities  in  the  San  Francisco  district2.  The 
presence  of  radiolaria  or  other  siliceous  organisms  in  cherts  is 
not  conclusive  evidence  that  the  bulk  of  the  silica  is  of  organic 
origin,  and  Lawson  regards  the  California!!  cherts  as  mainly 
deposited  from  submarine  springs3.  They  shew  every  grada- 
tion from  an  isotropic  mass  of  amorphous  silica  to  a  holo- 
crystalline  aggregate  of  quartz-granules,  the  crystallization 
beginning  from  distinct  centres,  as  in  the  devitrification  of  a 
glass.  The  radiolaria  are  preserved  in  chalcedonic  silica. 

A  peculiar  type  of  siliceous  deposit  is  the  sinter  of  the  hot 
springs  and  geysers  of  the  Yellowstone  Park,  Iceland,  and 
New  Zealand.  Mr  Weed4  has  shewn  that  this  material,  con- 
sisting of  colloid  silica,  is  in  great  part  secreted  by  filamentous 
algae  (Leptothrix,  etc.).  The  resulting  sinter  or  'geyserite'  does 
not  always  shew  clear  organic  structures.  Sinter  is  formed 
also  in  the  same  places  by  the  evaporation  of  the  water  in 
which  the  silica  was  carried  in  alkaline  solution. 

1  Edgworth  David  and  Pittman,  Q.  J.  G.  S.  (1899)  Iv,  33,  34 ;  Hinde, 
ibid.  38-42,  pi.  vra,  ix. 

2  Eansome  (and  Hinde),  Bull.  Dep.  Geol.  Univ.  Gal  (1894)  i,  198-200, 
235-237 ;  Lawson,  Amer.  Geol.  (1895)  xv,  348,  349,  and  15th  Ann.  Rep. 
U.  S.  Geol.  Sur.  (1895)  422-426. 

3  (7/.,  however,  Fairbanks,  Journ.  of  Geol.  (1897)  v,  65-68. 

4  A.  J.  S.  (1889)  xxxvii,  351 ;  more  fully  in  9th  Ann.  Rep.  U.  S.  Geol. 
Sur.  (1890)  pp.  613-676. 


E.   METAMORPHISM. 


USING  the  term  'metamorphism'  in  a  broad  sense,  we 
understand  by  it  the  production  of  new  minerals,  or  new 
structures,  or  both,  in  pre-existing  rock-masses.  We  must 
limit  such  a  conception  by  supposing  on  the  one  hand  that 
the  changes  produced  are  sufficient  to  give  a  distinctive  new 
character  to  the  rock  as  a  whole,  and  on  the  other  hand  that 
they  do  not  involve  the  loss  of  individuality  of  a  rock-mass 
(e.g.  bodily  fusion  must  be  excluded). 

It  is  customary  to  distinguish  thermal  metamorphism,  due 
to  heat,  and  dynamic  metamorphism,  due  to  pressure.  These 
can  to  some  extent  be  considered  separately,  and  we  shall 
examine  some  of  their  results  in  the  following  pages.  But, 
before  doing  so,  we  must  notice  that  very  important  changes, 
which  cannot  reasonably  be  excluded  from  the  domain  of 
metamorphism,  are  set  up  in  rock-masses  without  the  inter- 
vention of  either  high  temperature  or  great  mechanical  force. 
Many  of  these  changes  depend  upon  the  access  of  circulating 
waters  in  communication  with  the  atmosphere,  and  we  may, 
if  we  please,  roughly  group  them  as  meteoric  or  atmospheric 
metamorphism.  In  most  cases,  however,  these  processes  involve 
some  change  in  the  total  composition  of  the  rocks  affected, 
either  a  loss  of  some  constituents  or  an  addition  of  others 
(water,  oxygen,  carbonic  acid,  and  other  substances) :  in  other 
words  there  is  often  metasomatism  as  well  as  metamorphism. 

The  common  weathering-products  of  igneous  rocks  are 
results  of  such  processes,  but  it  is  convenient,  as  already 


288  METASOMATIC   CHANGES   IN   ROCKS. 

remarked,  to  restrict  the  term  metamorphism  to  cases  in 
which  the  general  mass  of  a  rock  is  considerably  altered :  the 
serpentine-rocks  are  an  example.  It  is  important  to  observe, 
however,  that  the  minerals  produced  by  secondary  actions  of 
the  kind  here  contemplated  include  some  which  are  also  com- 
mon as  original  constituents  of  igneous  rocks :  we  have  already 
mentioned  the  occurrence  in  this  way  of  secondary  quartz, 
felspars,  hornblende,  etc.  There  is  a  frequent  tendency  of  the 
new-formed  substance  to  form  as  a  crystalline  extension  of 
pre-existing  crystals  or  grains  of  the  same  mineral  (like  the 
quartz  in  many  quartzites) ;  or  again  for  a  pre-existing  mineral 
to  be  extended  by  an  outgrowth  of  some  allied  mineral  with 
the  same  crystalline  orientation:  e.g.  one  kind  of  plagioclase 
felspar  may  receive  an  extension  of  another  kind,  augite  of 
hornblende,  allanite  of  epidote. 

The  most  striking  examples  of  what  we  have  termed  atmo- 
spheric metamorphism  and  metasomatism  are  found  among  the 
sedimentary  rocks.  We  have  already  remarked  the  conversion 
of  sandstones  to  quartzites,  the  recrystallization  of  limestones ' 
and  their  replacement  by  dolomite,  iron-compounds,  silica,  etc., 
and  we  have  seen  that  very  many  argillaceous  sediments  have 
undergone  extensive  or  almost  complete  reconstitution  since 
they  became  strata. 

More  important  is  the  evidence  of  the  formation  of  crystal- 
line schists  on  an  extensive  scale  by  metasomatic  changes  alone 
described  by  Prof.  Van  Hise  in  the  Lake  Superior  region.  In 
the  upper  part  of  the  Penokee  Iron-bearing  Series2  of  Michigan 
and  Wisconsin  felspathic  grits,  greywackes,  etc.,  are  traced 
into  finely  crystalline  mica-schists,  with  biotite  and  muscovite, 
all  relics  of  the  clastic  structure  being  finally  obliterated.  In 
the  lower  members  of  the  same  series3  rocks  consisting  of 

1  Stefani  attributes  to  the  influence  of  circulating  waters  the  forma- 
tion from   Triassic  limestones   of  the  famous  Carrara  marble  in  the 
Apuan  Alps  :  see  G.  M.  1890,  372,  373  (Abstr.). 

2  Van  Hise,  A.  J.  S.  (1886)  xxxi,  453-459 ;   Irving  and  Van  Hise, 
Penokee  Iron-bearing  Series  in  IQth  Ann.  Eep.  U.  S.  Geol.  Sur.  (1890) 
423-435,  pi.  XXXVIII-XLII. 

3  Van  Hise,  A.  J.  S.  (1889)  xxxvii,  32-47 ;  Irving  and  Van  Hise,  I.e. 
Gf.  Hudleston,  Pr.  Geol.  Ass.  (1889)  xi,  133-138. 


MINERALOGICAL  AND  STRUCTURAL  REARRANGEMENTS.     289 

impure  carbonates  mixed  with  chert  have  been  converted  into 
ferruginous  quartz-schists,  magnetite-  and  haematite-schists, 
magnetite-  and  haematite-bearing  actinolite-schists,  etc.,  also  by 
metasomatic  processes  (silicification  and  other  replacements), 
apparently  without  the  conditions  of  either  thermal  or  dynamic 
metamorphism.  Similar  rocks  occur  in  the  Mesabe  range, 
Minnesota1. 

We  may  now  pass  on  to  such  changes  affecting  rock-masses 
as  are  more  usually  understood  by  the  term  metamorphism  as 
employed  in  text-books.  The  changes  are  in  part  mineral- 
ogical  (in  most  cases  without  any  very  important  metasom- 
atism), in  part  structural.  These  two  lines  of  change  are  so 
connected  that  they  cannot  be  considered  quite  separately: 
roughly  we  may  say  that  mineralogical  modifications  are  the 
more  prominent  in  thermal  metamorphism,  and  structural  in 
dynamic. 

While  treating  in  turn  the  chief  features  of  thermal  and  of 
dynamic  metamorphism,  we  must  remember  that  their  effects 
may  be  associated  or  superposed  in  the  same  area,  and  the 
assigning  of  particular  mineralogical  changes  to  one  or  the 
other  cause  is  in  many  cases  still  a  question  for  discussion. 

1  Bayley,  A.  J.  S.  (1893)  xlvi,  176. 


H.  P.  19 


CHAPTER  XX. 

THERMAL  METAMORPHISM. 

UNDER  this  head  we  include  all  changes  produced  in  pre- 
existing rock-masses  by  the  influence  of  high  temperature1. 
In  the  simplest  case  this  is  brought  about  by  the  intrusion  of 
an  igneous  magma  in  the  neighbourhood  ('contact'  or  'local' 
metamorphism  of  many  authors);  but  we  must  also  include 
the  effects  of  heat  mechanically  generated  (thermal  being  then 
associated  with  dynamic  phenomena),  and  those  due  to  the 
internal  heat  of  the  Earth  in  a  rise  of  the  isogeotherms. 
These  latter  especially  may  affect  rock-masses  on  a  regional 
scale.  We  shall  here  avoid  complication  by  drawing  our 
examples,  so  far  as  is  possible,  from  cases  of  thermal  meta- 
morphism produced  by  igneous  intrusions. 

Characteristic  minerals.  It  will  be  convenient  to 
refer  briefly  to  the  commoner  minerals  formed  in  thermal 
metamorphism,  some  of  them  being  unknown  or  rare  in  igneous 
rocks.  Quartz  and  felspars  are  widely  distributed  in  meta- 
morphic  rocks  of  various  kinds.  The  felspars  include  orthoclase, 
albite,  anorthite,  and  various  intermediate  members  of  the 
plagioclase  series.  They  are  often  perfectly  clear,  and  when  they 
occur  as  minute  shapeless  granules  in  a  mosaic  they  may  easily 
be  mistaken  for  quartz  without  special  optical  tests2.  The 

1  For  a   discussion  of  various  questions  concerning  thermal  meta- 
morphism see  Science  Progress  (1894),  ii,  185-201,  290-303. 

2  Becke  has  given  a  staining  method,  using  aniline  blue  after  etching 
with  hydrofluoric  acid.     Plagioclase  is  deeply  coloured,  orthoclase  only 
slightly  affected,  and  quartz  unchanged  (for  examples  see  Berwerth, 
Lief.  in). 


MINERALS  OF  THERMAL   METAMORPHISM.  291 

larger   grains    shew   cleavage    and   sometimes    characteristic 
twinning  or  some  approach  to  crystal  outline. 

Both  muscovite  and  biotite  are  found  in  metamorphosed 
rocks,  the  latter  being  very  widely  distributed.  It  is  appar- 
ently a  haughtonite  and  always  strongly  pleochroic,  with  a 
deep  reddish-brown  colour  or,  for  vibrations  parallel  to  the 
cleavage-traces,  a  very  deep  brown  with  a  noticeable  greenish 
tone.  Intensely  pleochroic  haloes  surround  certain  inclusions. 
Less  usual  than  brown  mica  as  a  conspicuous  mineral  is  a 
green  ripidolite  or  a  yellowish  or  greenish  chlorite.  In  the 
fine-textured  'base'  of  argillaceous  rocks,  however,  Mr  Hutch- 
ings1  notes  that  the  conversion  of  impure  micaceous  material 
into  an  aggregate  of  muscovite  and  chlorite,  so  characteristic 
in  the  passage  of  clays  and  shales  into  slates,  is  also  met  with 
in  thermal  metamorphism,  especially  where  there  is  no  abund- 
ant production  of  biotite.  Exceptionally  we  find  the  manganese- 
bearing  chloritoid  mineral  ottrelite*  (fig.  69,  A).  It  builds 
flakes  without  special  orientation,  and  freely  encloses  impurities : 
the  lamellar  twinning  parallel  to  the  base  and  a  modification  of 
hour-glass  structure  are  noticeable3. 

Highly  characteristic  of  the  metamorphism  of  argillaceous 
and  some  other  rocks  are  silicates  rich  in  alumina.  Andalusite 
forms  more  or  less  idiomorphic  crystals  with  the  prism-form 
and  usually  some  traces  of  the  prismatic  cleavage.  It  is  re- 
cognized by  its  moderately  high  refractive  index  with  low 
double  refraction  (about  the  same  as  in  labradorite)  and 
straight  extinction.  When  it  shews  any  colour,  it  is  pleochroic, 
giving  a  rose  tint  for  longitudinal  and  a  very  faint  green  for 
transverse  vibrations.  It  may  be  quite  clear,  or  may  contain 
numerous  inclusions,  certain  enclosed  minerals  being  sur- 
rounded by  a  pleochroic  halo  (bright  yellow  to  colourless).  In 
chiastolite4  the  elongated  crystals  contain  a  large  amount  of 
foreign  matter,  apparently  carbonaceous,  arranged  in  the 
fashion  peculiar  to  the  mineral  (fig.  68).  Sillimanite  (fibrolite) 

1  G.  M.  1896,  344,  345 ;  1898,  74,  75. 

2  See  Hutchings,  G.  M.   1889,  214;   Whittle,  A.  J.  S.  (1892)  xliv, 
270-277. 

3  Cohen  (3),  pi.  xxn,  fig.  4 ;  xxx,  fig.  3. 

4  Ibid.  pi.  xvn,  fig.  4. 

19—2 


292  MINERALS   OF   THERMAL    METAMORPHISM. 

builds  elongated  prisms  or  needles,  which  in  shape,  cross- 
fracture1,  and  refractive  index  resemble  apatite,  but  have  a 
much  stronger  birefringence  (fig.  66).  They  are  often  crowded 
together  in  matted  aggregates  imbedded  in  quartz  ('Faser- 
kiesel'  or  'quartz  sillimanitisd ' 2).  CyomW  or  disthene  is  found 
less  commonly,  building  more  or  less  rounded  crystals  or  grains, 
with  rjinacoidal  cleavage  and  a  cross-fracture  corresponding  with 
a  gliding-plane.  In  thin  sections  it  is  colourless  or  pale  blue, 
with  pleochroism,  and,  owing  to  its  high  refractive  index, 
shews  a  strong  relief.  Longitudinal  sections  give  extinction- 
angles  up  to  31°.  Staurolite  forms  good  crystals,  the  larger 
ones  always  crowded  with  various  inclusions4.  When  fresh, 
it  is  yellowish  or  reddish-brown  with  distinct  pleochroism 5  and 
strong  refringence  and  birefringence.  This  mineral,  however, 
and  in  varying  degree  all  the  aluminous  silicates,  are  very 
liable  to  decomposition,  the  characteristic  product  being  white 
mica  in  minute  scales  (the  ' shimmer-aggregate'  of  Barrow6). 
Cordierite  is  sometimes  less  easily  recognized.  It  is  commonly 
in  shapeless  grains  crowded  with  inclusions,  but  sometimes 
builds  pseudo-hexagonal  prisms,  basal  sections  of  which  oc- 
casionally shew  the  curious  triple  twinning7.  The  mineral 
rarely  shews  its  colour  and  pleochroism  in  thin  slices,  but  is 
sometimes  stained  of  a  yellow  tint.  The  refractive  index  and 
double  refraction  are  low. 

The  metamorphism  of  calcareous  rocks  gives  rise  to  numer- 
ous silicates  rich  in  lime,  or  in  lime  and  magnesia.  The  pure 
lime-silicate  wollastonite  is  colourless  in  thin  slices,  and  shews 
lower  refringence  and  birefringence  than  the  augites.  It  is 
further  distinguished  by  having  its  two  principal  cleavages 
and  its  direction  of  elongation  perpendicular  to  its  plane  of 
symmetry,  and  consequently  giving  straight  extinction.  As  a 

1  Cohen  (3),  pi.  XLVII,  fig.  2. 

2  Ibid.  pi.  xxxvn,  fig.  3;  Williams,  A.  J.  S.  (1888)  xxxvi,  pi.  vi, 
figs.  2,  4;  Barrow,  Q.  J.  G.  S.  (1893)  xlix,  338,  pi.  xvi,  figs.  1,  2. 

3  Cohen  (3),  pi.  XLII,  fig.  4 ;  Barrow,  I.e.,  338,  339,  pi.  xvi,  figs.  3,  4. 

4  Williams,  A.  J.  S.  (1888)  xxxvi,  pi.  vi,  fig.  3.     On  arrangement  of 
inclusions  see  Penfield  and  Pratt,  ibid.  (1894)  xlvii,  81-89. 

5  Cohen  (3),  pi.  xxvii,  fig.  1. 

6  Q.  J.  G.  S.  (1893)  xlix,  340,  pi.  xvi,  fig.  5. 

7  Cohen  (3),  pi.  xxix,  fig.  3. 


MINERALS  OF  THERMAL   METAMORPHISM.  293 

rule,  it  occurs  in  quite  small  imperfect  crystals.  The  augites 
of  metamorphosed  limestones,  etc.,  are  either  non-aluminous 
(diopside)  or  aluminous  (pmphacite).  They  build  imperfect 
crystals  or  crystalline  patches,  take  part  in  a  finely  granular 
mosaic,  or  occur  as  little  globules  enclosed  in  other  minerals. 
The  crystals  are  occasionally  twinned  on  the  usual  law.  The 
green  colour  is  often  imperceptible  in  thin  slices.  Both  diopside 
and  omphacite  give  extinction-angles  of  38°  or  40°,  and  it  is 
not  always  possible  to  discriminate  them,  though  the  former  is 
sometimes  betrayed  by  its  partial  conversion  into  serpentine. 
The  most  common  amphibole  in  these  rocks  is  a  colourless 
tremolite  in  imperfecfc  crystals,  crystalline  patches,  veins,  or 
sheaf-like  groupings.  It  may  shew  a  fibrous  structure  or  a 
good  hornblende-cleavage,  and  a  rough  cross-fracture  is  also 
common.  Green  hornblende  and  blade-like  actinolite  are  found 
in  some  rocks.  The  lime-garnet  grossularite  forms  well-bounded 
crystals,  often  of  considerable  size,  with  included  pyroxene 
granules  (fig.  72).  It  is  often  feebly  birefringent,  and  further 
shews  between  crossed  nicols  a  polysynthetic  twinning  of  a  re- 
markable kind1.  With  this  structure  goes  a  strongly  marked 
zonary  banding,  the  concentric  zones  differing  in  birefringence. 
Idocrase  occurs  either  in  well-built  crystals  or  in  shapeless 
plates  enclosing  other  minerals.  The  cleavage  and  colour  are 
usually  not  to  be  observed  in  thin  sections.  The  birefringence 
is  variable,  and  a  crystal  often  shews  bands  or  lamellae  differing 
in  interference-colours.  Zoisite  occurs  in  little  prisms  often 
grouped  in  sheaf-like  fashion.  It  is  characterized  by  longi- 
tudinal cleavage-traces,  high  refractive  index,  low  polarization- 
tints,  and  straight  extinction.  Epidote,  often  associated  with 
the  last-named  mineral,  is  usually  in  shapeless  grains  or 
granular  aggregates,  though  it  may  present  crystal-boundaries 
towards  calcite,  etc.  The  cleavages  are  well-marked,  the  two 
sets  of  traces  intersecting  at  about  65°  in  a  cross-section. 
Twinning  is  uncommon.  The  larger  crystals  shew  the  yellow 
colour  and  pleochroism.  Other  distinctive  characters  are  the 
high  refractive  index,  very  brilliant  polarization-tints,  and 
straight  extinction  in  longitudinal  sections. 

1  Cohen  (3),  pi.  LII;  also,  for  numerous  figures,  Klein,  Neu.  Jahrb. 
1883,  i,  pi.  vn-ix. 


294  MINERALS   OF  THERMAL  METAMORPHISM. 

A  characteristic  mineral  in  metamorphosed  dolomite-rocks 
is  the  pure  magnesian  olivine  forsterite.  It  forms  either 
crystals  of  tabular  habit  (fig.  73)  or  rounded  grains,  and  by 
alteration  gives  rise  to  serpentine.  In  certain  cases  magnesia 
has  crystallized  in  the  form  of  periclase,  in  octahedra,  or  in 
rounded  grains  (e.g.  in  blocks  ejected  from  Monte  Somma, 
Vesuvius) ;  but  this  passes  readily  by  hydration  into  brucite,  a 
clear,  colourless  mineral  with  one  (basal)  cleavage,  straight 
extinction  with  the  cleavage-traces,  low  refringence,  and  strong 
birefringence  (nearly  equal  to  that  of  augite). 

Among  other  products  of  thermal  metamorphism  in  various 
rocks  may  be  mentioned  common  garnet,  chloritoid,  dipyre, 
magnetite  and  ilmenite,  pyrite  and  pyrrhotite,  sphene,  rutile 
and  anatase,  spinels,  corundum,  and  graphite.  Further,  the 
formation  of  a  certain  amount  of  isotropic  matter  is  charac- 
teristic in  some  cases1. 

As  a  special  mineral  formed  in  metamorphosed  rocks  near 
an  igneous  intrusion  may  be  noticed  tourmaline.  This  mineral 
occurs  in  little  grains,  often  in  veins  which  represent  cracks, 
or  sometimes  very  abundantly  as  a  constituent  of  a  kind  of 
contact-breccia.  It  is  restricted  to  the  neighbourhood  of  acid 
intrusions,  and  depends  on  an  actual  introduction  of  certain 
materials  from  the  igneous  magma.  White  mica  has  some- 
times a  similar  occurrence. 

Metamorphism  of  arenaceous  rocks.  The  effects  of 
thermal  metamorphism  in  arenaceous  rocks  are  simple  or  com- 
plex according  to  the  homogeneous  or  heterogeneous  nature  of 
the  deposits  affected.  In  a  pure  quartz-sandstone  or  quartzose 
grit  there  are  no  degrees  of  metamorphism  possible.  If  the 
temperature  be  sufficiently  high,  the  whole  will  be  recrystal- 
lized  into  a  clear  quartz-mosaic  without  a  trace  of  the  original 
clastic  character.  Short  of  this  change,  the  sandstone  will  be 
unaltered,  except  in  such  minor  points  as  the  expulsion  of  the 
water  from  the  fluid-pores  of  the  quartz,  an  effect  noticed  by 
Sorby  at  Salisbury  Crags.  The  homogeneous  quartzite  result- 
ing from  the  complete  metamorphism  of  a  pure  quartzose  rock 

1  On  this  and  some  other  points  see  Hutchings,  G.  M.  1894,  36-45, 
64-75. 


METAMORPHOSED   SANDSTONES.  295 

is  not  difficult  to  distinguish  from  a  quartzite  formed  by  the 
deposition  of  interstitial  quartz.  There  is  no  distinction  of 
original  grains  and  cementing  material,  no  secondary  growth 
upon  original  nuclei,  but  each  element  of  the  mosaic  is  clear 
and  homogeneous,  presenting  an  irregular  boundary  which  fits 
into  the  inequalities  of  the  adjoining  elements.  Such  quartz- 
ites  are  locally  produced  in  the  Skiddaw  grits  abutting  on  the 
large  granophyre  mass  at  Ennerdale,  in  the  Carboniferous 
sandstones  near  the  Whin  Sill  of  Teesdale,  and  in  many  other 
places. 

If  the  original  sediment  contained  felspar  grains,  not  much 
altered,  as  well  as  quartz,  the  felspar  is  recrystallized  with  the 
quartz,  and  without  careful  examination  is  liable  to  be  over- 
looked in  the  resulting  mosaic. 

Where  a  quartzose  sandstone  or  grit  has  contained  scattered 
decomposition-products,  such  as  kaolin,  calcite,  and  chloritic 
minerals,  in  small  quantity,  metamorphism  produces  a  quartzite 
with  granules  of  some  accessory  mineral.  Thus,  near  the 
Shap  granite,  the  grits  in  the  Coniston  Flags  group  have  been 
transformed  into  a  quartzite  with  granules  of  colourless  pyro- 
xene, formed  from  kaolin  and  calcite.  Similarly  the  chloritic 
minerals  give  rise  to  brown  mica.  A  curious  green  mica  occurs 
in  the  quartzite  of  Clova  in  Forfarshire. 

The  metamorphism  of  a  specially  pure  type  of  siliceous 
rock  has  been  described  by  Mr  Home1  in  the  case  of  the 
Arenig  radiolarian  cherts  of  the  south  of  Scotland,  as  they 
approach  the  Loch  Doon  granite.  The  final  result  is  a  mosaic 
of  granular  quartz  with  numerous  minute  round  inclusions  of 
biotite. 

If  the  original  rock  was  more  impure,  containing  plenty  of 
aluminous  and  other  substances,  the  product  of  metamorphism 
ceases  to  have  any  apparent  resemblance  to  a  quartzite. 
Silicates  of  alumina,  garnet,  micas,  etc.,  may  be  extensively 
produced,  and  the  metamorphosed  rock  assume  the  aspect  of 
a  fine  or  even  a  coarse  gneiss  (fig.  66).  Remarkable  examples 
are  presented  by  the  Silurian  grits  and  flags  round  the  Old 

1  Rep.  Brit.  Ass.  for  1892,  712.  Cf.  Ann.  Rep.  Geol.  Sur.  for  1896, 
46,  and  Teall,  Mem.  Geol.  Sur.,  Silur.  Rocks  Scot.  (1899)  640-642, 


296 


MICA-   AND   SILLIMANITE-SCHISTS. 


Red  Sandstone  granites  of  Galloway1.  Here  the  chief  con- 
stituents are  quartz,  muscovite,  a  deep  brown  biotite,  and 
red  garnet  (colourless  in  slices),  felspar  being  only  subordinate. 


9 


Fia.  66.     GARNET-SILLIMANITE-SCHIST  OB  GNEISS,  A  HIGHLY 

METAMORPHOSED   GRIT,    CLOVA,    FoRFARSHIRE  ;       X  20. 

The  right  half  of  the  figure  shews  an  area  of  clear  quartz  full  of  little 
prisms  of  sillimanite  with  characteristic  cross- fracture  (sq) :  to  the  left 
are  clear  quarts;  (q),  biotite  (6i),  and  part  of  a  large  garnet  (g)  [1808]. 

The  garnets,  except  at  the  margin  of  each  crystal,  are  crowded 
with  minute  granular  inclusions :  they  tend  to  occur  in  clusters 
moulded  by  clear  quartz,  a  frequent  association  in  many  meta- 
morphic  rocks.  Nearer  to  the  granite  the  texture  of  the  rock 
becomes  coarser,  and  the  muscovite  and  quartz  are  seen  to  be 
crowded  with  narrow  needles  of  sillimanite  up  to  '01  inch  long. 
The  same  minerals  as  before  are  present,  with  a  few  crystals 
of  plagioclase  and  rarely  a  little  brown  tourmaline.  At  a 
hundred  yards  from  the  granite  margin  the  texture  is  very 
coarse,  the  abundant  white  mica  building  plates  half  an  inch 
in  length  and  relatively  thick.  Dense  matted  aggregates  of 

1  Miss  Gardiner,  Q.  J.  G.  S.  (1890)  xlvi,  569-580 ;  Teall,  Mem.  Geol. 
Sur.,  Silur.  Rocks  Scot.  (1899)  644-647. 


SILLIMANITE-   AND   CORDIERITE-GNEISSES. 


297 


sillimanite  needles  occupy  the  interior  of  the  quartz  and  mus- 
covite,  leaving  the  borders  of  the  crystals  clear.  Some  of  the 
most  altered  rocks  shew  bands  or  streaks  rich  in  particular 
minerals,  such  as  lenticular  patches  of  garnet  set  in  clear 
quartz  or  streaks  composed  essentially  of  muscovite  and  silli- 
manite, dark  mica  being  less  plentiful  (fig.  67). 


ms 


FIG.  67.    GARNET-SILLIMANITE-SCHIST  OB  GNEISS,  A  HIGHLY  META- 
MORPHOSED GRIT,  KNOCKNAIRLING  HILL,  NEW  GALLOWAY;     x20. 

The  figure  shews  portions  of  two  lenticular  streaks,  one  consisting 
essentially  of  muscovite  crowded  with  minute  needles  of  sillimanite  (ms), 
the  other  of  garnet  (g]  set  in  clear  quartz  (q)  [1173]. 

Some  highly  metamorphosed  sediments  in  the  eastern 
Highlands  of  Scotland  are  rich  in  cordierite,  usually  crowded 
with  inclusions  of  other  minerals  and  having  round  certain 
inclusions  the  characteristic  pleochroic  yellow  haloes.  An 
example  from  the  Buck  of  Cabrach  in  BanfFshire  consists  of 
cordierite  and  white  mica  in  allotriomorphic  crystals  and  a 
mosaic  of  microcline,  with  some  quartz,  andalusite,  magnetite, 
and  biotite.  This  rock  has  a  massive  structure,  but  others  in 
the  same  district  are  gneissose  and  schistose1.  Corundum, 

1  Teall,  Mem.  Geol.  Sur.  Scot.,  ExpL  Sheet  75  (1896),  36,  37,  45 ;  and 
Ann.  Rep.  Geol.  Sur.  for  1896,  18,  19. 


298     METAMORPHOSED   SLATES:    CHIASTOLITE-SLATES. 

as  well  as  sillimanite,  spinel,  etc.,  occurs  in  some  of  these 
cordierite-bearing  rocks1. 

Metamorphism  of  argillaceous  rocks.  The  effects 
of  thermal  metamorphism  in  clays,  shales,  or  slates  depend  in 
the  early  stages  of  alteration  on  the  mineralogical,  and  in  the 
later  stages  on  the  chemical,  composition  of  the  rocks  affected. 

In  strata  containing  carbonaceous  matter,  this  is  one  of 
the  first  ingredients  to  suffer  change.  It  is  either  dissipated 
and  expelled  or  converted  into  graphite.  The  latter  is  in 
some  cases  aggregated  into  little  dark  spots,  producing  one 
type  of  what  is  known  as  'spotted  slate'  (Ger.  Knotenschiefer). 
This  peculiarity  may  be  seen  in  otherwise  unaltered  strata, 
and  it  disappears  with  advancing  metamorphism.  The  minute 
needles  of  rutile  so  abundant  in  slates  also  seem  to  be  rather 
readily  affected,  giving  place  to  stouter  crystals  of  the  same 
mineral,  or  less  commonly  to  anatase  or  brookite.  Another 
early  effect  of  metamorphism  is  the  production  of  little  flakes 
of  brown  mica  (probably  the  haughtonite  variety  of  biotite) 
from  chloritic  substances,  etc.  With  this  there  may  be  a 
crystallization  of  iron-ores  (magnetite  or  pyrites).  In  some 
cases  a  chloritic  mineral  or  ottrelite  is  formed  instead  of  the 
mica.  In  rocks  rich  in  alumina  chiastolite  is  produced  con- 
currently with  biotite2,  e.g.  in  the  Skiddaw  district  (Banner- 
dale,  Roughton  Gill,  etc.,  fig.  68). 

With  advancing  metamorphism  graphitic  spots  and  chiast- 
olite-crystals  are  lost,  and  the  metamorphism  begins  to  affect 
the  whole  body  of  the  rock,  the  chief  products  formed  being 
usually  quartz  and  biotite.  Of  these  the  latter  often  has  its 
flakes  oriented  in  accordance  with  the  original  lamination  or 
cleavage  of  the  rock,  and  we  have  thus  one  type  of  mica-schist 
(Ger.  Glimmerschiefer).  These  rocks  may  have  no  trace  of 
the  original  clastic  nature  of  the  deposit,  except  perhaps  some 
minute  angular  quartz-grains.  They  sometimes  shew  a  spotted 
character  quite  different  from  that  mentioned  above,  and  con- 
sisting in  little  ovoid  spaces  free,  or  relatively  free,  from  the 

1  Teall,  Summary  of  Progress  Geol.  Sur.  for  1898,  86-88 ;  Pr.  GeoL 
Ass.  (1899)  xvi,  63,  64  (Monadh  Driseag  near  Loch  Awe). 

2  For  good  coloured  figures  see  Teall,  pi.  xxxm,  fig.  2  (Skiddaw) ; 
Fouque  and  LeVy,  pi.  in,  fig.  1  (Brittany). 


ANDALUSITE-MICA-SCHISTS. 


299 


flakes  of  biotite  which  crowd  the  rest  of  the  rock.  Such 
spaces  often  shew  distinctly  crystalline  properties,  giving 
extinction  parallel  with  their  length,  and  in  many  cases, 
at  least,  they  are  ill-developed  crystals  of  andalusite.  They 


FIG.  68.     CHIASTOLITE-SLATE,  SKIDDAW  SLATE  METAMORPHOSED 
BY  GRANITE,  BANNERDALE,  SKIDDAW  ;     X  20. 

Besides  the  good  cross-sections  of  chiastolite  (ch),  shewing  character- 
istic arrangement  of  enclosed  impurities,  there  are  imperfectly  developed 
crystals  (ch')  clearly  detected  by  using  polarized  light.  In  the  general 
mass  of  the  rock  the  chief  metamorphic  effect  is  the  production  of  little 
flakes  of  biotite  (6)  [1111]. 

may  be  observed  in  the  Skiddaws  of  the  Caldew  and  Glendera- 
terra  valleys.  When  andalusite  is  better  developed,  it  appears 
in  clear  crystal-grains  or  in  crystal-plates  enclosing  other 
minerals  :  both  forms  are  seen  in  the  Skiddaw  district,  where 
andalusite-mica-schists  have  been  extensively  formed1.  Other 
minerals,  such  as  white  mica  and  little  garnets,  occur  more 
locally  (Sinen  Gill,  Grainsgill,  etc.). 


1  On  the  various  stages  of  metamorphism  in  the  Skiddaw  district,  see 
especially  Kosenbusch  (translated)  in  Naturalist,  1892,  119,  120. 


300 


CORDIERITE-MICA-SCHISTS. 


As  another  example  of  well-marked  spots  due  to  the 
development  of  imperfect  crystals,  we  may  take  the  Coniston 
Flags  near  their  contact  with  the  Shap  granite1.  Here  the 
spots  are  small  and  ovoid,  with  numerous  inclusions,  but  give 
a  distinctly  crystalline  reaction,  the  essential  mineral  ex- 
tinguishing parallel  to  the  length  of  the  spots.  Mr  Hatchings 
finds  it  to  be  cordierite2  (fig.  69,  B}.  The  same  mineral  forms 


B 


FIG.  69;     x  20. 

A.  Ottrelite-slate  (metamorphosed  Cambrian  slate),  Ottre,  Ardenne, 
Belgium.     Crystals  of  ottrelite  crowded  with  inclusions  [1565]. 

B.  Cordierite-Mica-schist    (metamorphosed    Coniston   Flags),   near 
Shap  granite,  Wasdale  Beck,  Westmorland.     The  ovoid  spaces  free  from 
biotite  indicate  imperfect  crystals  of  cordierite  [866]. 

somewhat  larger  spots  in  some  of  the  metamorphosed  Skiddaw 
Slates  of  the  Caldew  valley,  and  here  some  of  the  imperfect 
crystals  shew  the  characteristic  composite  twinning  (Swineside)3. 
Various  types  of  spotted  and  flecked  rocks  due  to  meta- 
morphism  have  been  styled  spilosite,  Fleckschiefer,  Frucht- 
schiefer,  Garbenschiefer,  etc.,  and  shew  spots  and  patches  of 

1  Q.  J.  G.  S.  (1891)  xlvii,  320,  pi.  xn,  fig.  5. 

2  Hutchings,  G.  M.  1894,  65. 

3  G.  M.  1894,  169. 


'SPOTTED   SLATES.' 


301 


very  various  dimensions.  In  some  they  are  evidently  ill-formed 
crystals  (e.g.,  cordierite) ;  in  others  the  true  nature  of  the  spots 
is  not  very  clearly  understood.  Mr  Teall1  compares  with  the 
typical  'spilosite'  of  the  Harz  some  slates  near  Tremadoc 


FIG.  70.     ANDALUSITE-MICA-SCHIST,  METAMORPHOSED  SKIDBAW  SLATE, 

CLOSE    TO    GRANITE,    SlNEN    GlLL,    SlUDDAW  J       X  20. 

The  rock  consists  of  andalusite,  biotite,  and  quartz,  with  subordinate 
muscovite  and  magnetite.  It  has  not  a  very  marked  schistose  character, 
and  would  be  styled  Hornfels  by  the  German  writers.  All  the  lower  half 
of  the  figure  is  occupied  by  a  large  crystal -plate  of  andalusite,  enclosing 
numerous  flakes  of  mica  and  needles  of  sillimanite  [1446]. 

altered  by  large  sheets  of  diabase.  Here  the  spots  are  almost 
invisible  in  a  slice  viewed  in  ordinary  light,  but  become 
conspicuously  dark  between  crossed  nicols.  This  seems  to  be 
due  to  numerous  minute  overlapping  scales  of  chlorite.  A 
micaceous  mineral  occurs  more  sparingly,  and  an  aggregate 
of  granules  having  the  refraction  and  double  refraction  of 
quartz  and  felspar.  Similar  phenomena  are  seen  in  other 
parts  of  North  Wales,  e.g.,  near  the  granite  of  Ffestiniog,  and 
in  many  other  countries2.  Other  metamorphosed  slates  near 

1  Brit.  Petr.  218. 

2  Clements  describes  a  rock  of  this  type  in  metamorphosed  Huronian 
slates  near  Mansfield,  Mich. ;  A.J.S.  (1899)  vii,  86. 


302  GARNETIFEROUS  MICA-SCHISTS. 

Tremadoc  have  a  banded  rather  than  a  spotted  character,  thus 
answering  to  the  '  desmoisite '  rather  than  the  'spilosite'  of 
the  Harz  geologists. 

In  extreme  cases  of  metamorphism  the  rocks  lose  all 
spotted,  and  frequently  all  banded  and  schistose,  structures, 
passing  sometimes  into  an  extremely  compact,  fine-textured 
mass  of  quartz,  micas,  iron-ores,  etc.  (Ger.  Hornfels,  Fr. 


FIG.  71.     GRAPHITIC  MICA-SCHIST,  BLAIR  ATHOLL,  PERTHSHIRE  ; 
CUT  PERPENDICULARLY  TO  THE  SCHISTOSITY  ;     X  20. 

The  rock  consists  mainly  of  quartz  and  sericitic  mica,  with  some 
finely  divided  graphite.  There  are  also  numerous  dodecahedra  of  garnet, 
each  in  the  centre  of  a  lenticular  streak  or  « eye  '  of  quartz  [1834]. 

corne'enne,  '  hornstone '  of  some  writers).  Andalusite,  garnet, 
etc.,  characterize  different  types  (Ger.  Andalusithornfels, 
Granathornfels,  etc.).  Some  highly  metamorphosed  strata, 
however,  have  a  marked  schistose  character,  usually  due  to 
micas  of  sericitic  habit  following  old  structural  planes  in  the 
rock.  Dark  mica  usually  predominates,  but  white  is  also 
frequent.  Red  garnet  is  common  in  mica-schists  of  this  kind, 
and  other  minerals  may  occur,  according  to  the  original 
chemical  composition  of  the  rock.  A  well-marked  zone  of 
graphitic  mica-schists  is  known  in  the  Central  Highlands,  and 


MUSCOVITE-SCHISTS.  303 

shews  the  characters  of  a  thermally  metamorphosed  rock 
(fig.  71).  The  graphite  doubtless  represents  carbonaceous 
matter  of  organic  origin. 

The  Ordovician  slates  near  the  Leinster  granites  are  con- 
verted into  mica-schists  with  staurolite  and  graphite.  Locally 
they  shew  spots,  which  develop  into  crystals  of  andalusite, 
sometimes  of  considerable  size.  Patches  of  rock  enclosed 
by  the  granite  exhibit  a  higher  grade  of  metamorphism,  with 
development  of  staurolite,  garnet,  idocrase,  zinnwaldite  mica, 
tourmaline,  actinolite,  etc.1 

In  slates  which  originally  contained  a  considerable  amount 
of  muscovite  or  of  finely  divided  felspathic  matter,  or  at  least 
had  not  become  much  impoverished  in  alkalies,  the  phenomena 
of  metamorphism  are  somewhat  different  from  those  sketched 
above.  Chiastolite  is  not  formed,  and  andalusite  does  not 
usually  figure  largely  in  the  more  metamorphosed  rocks,  while 
new-formed  white  mica  occurs  abundantly  with  the  biotite  or 
to  its  exclusion.  A  good  example  of  the  type  characterized 
by  white  mica  is  afforded  by  the  slates  of  Charnwood  Forest 
near  a  granitic  intrusion  at  Brazil  Wood2.  Here  the  ragged 
flakes  of  muscovite  enclose  subordinate  biotite  with  parallel 
intergrowth :  a  chlorite  is  also  present,  besides  clear  quartz 
and  granules  of  opaque  iron-ore.  Near  the  Whin  Sill  of 
Teesdale  Mr  Hutchings3  finds  the  Lower  Carboniferous  shales 
converted  in  some  beds  to  a  minute  aggregate  wholly  of  musco- 
vite and  chlorite.  In  many  cases  of  contact-metamorphism 
material  introduced  into  the  metamorphosed  rocks  from  an 
invading  magma  has  given  origin  to  special  minerals  not 
dependent  on  the  nature  of  the  strata  affected.  The  com- 
monest of  these  special  minerals  is  tourmaline.  It  has  been 
formed  abundantly  in  many  of  the  slates  bordering  the  granitic 
intrusions  of  Cornwall4  and  Devon.  Besides  the  brown  or  blue 
tourmaline,  the  metamorphosed  rocks  consist  of  quartz,  micas, 

1  Watts,  Guide,  39,  40. 

2  Bonney,  Q.  J.  G.  S.  (1877)  xxxiii,  783.     These  slates  are  probably 
composed  in  great  part  of  volcanic  material. 

3  G.  M.  1896,  348-350  ;  1898,  74-77,  125-128. 

4  Allport,  Q.  J.  G.  S.  (1876)  xxxii,  408-417.    For  a  striking  instance 
see  Hawes  on  the  Albany  Granite,  N.  H.,  A.  J.  S.  (1881)  xxi,  21-32. 


304  ADINOLES. 

chlorite,  andalusite,  etc.  Some  of  the  less  altered  slates  have 
a  spotted  character  in  which  the  spots  are  imperfect  crystal- 
grains  of  andalusite.  The  more  altered  rocks  are  mica-schists. 

In  the  neighbourhood  of  some  basic  intrusions  there  seems 
to  have  been  more  important  metasomatic  change,  brought 
about  especially  by  a  transference  of  soda  from  the  magma  to 
the  rocks  undergoing  metamorphism.  Some  of  the  '  adinoles ' 
of  the  Harz  are  ascribed  to  this  action.  They  consist  essen- 
tially of  a  fine-textured  mosaic  of  quartz  and  albite  with 
sometimes  other  minerals.  Mr  Teall1  compares  with  adinole 
a  rock  at  Y  Gesell  near  Tremadoc,  which  has  the  same  mineral 
composition,  with  the  addition  of  minute  scales  of  mica  and 
chlorite.  In  an  adinole  near  a  diabase  intrusion  in  the 
Huronian  slates  of  Mansfield,  Michigan,  more  than  half  the 
rock  consists  of  albite,  the  other  chief  constituents  being 
actinolite  and  quartz2.  As  an  example  apparently  of  a  like 
transformation  in  arenaceous  and  even  siliceous  rocks,  we  may 
note  a  case  on  Angel  Island,  San  Francisco,  where,  according 
to  Ransome3,  not  only  felspathic  sandstones  but  even  radio- 
larian  cherts  are  converted  to  glaucophane-schists,  composed  of 
quartz,  albite,  glaucophane,  biotite,  etc. 

Apart  from  any  introduction  of  soda,  etc.,  a  very  im- 
portant feature  in  the  metamorphism  of  many  argillaceous 
rocks  is  the  abundant  new  formation  of  felspars.  This  is 
probably  a  quite  common  occurrence  in  the  advanced  stages 
of  metamorphism,  but  very  careful  study  is  needed  to  dis- 
tinguish the  felspar  from  quartz  when  it  occurs  in  a  minutely 
granular  mosaic.  Good  instances  are  furnished  by  the  Con- 
iston  Flags  near  the  Shap  granite.  In  shales  near  the  Whin 
Sill  of  Teesdale,  Mr  Hutchings4  finds  spherical  aggregates  of 
quartz  and  felspar  fibres. 

An  example  of  extreme  metamorphism  is  afforded  by  the 
Silurian  shales  near  the  New  Galloway  granite5.  The  rocks 

1  Brit.  Petr.  219-221. 

2  Clements,  A.  J.  S.  (1899)  vii,  87,  88. 

3  Bull.  Geol.  Dep.  Univ.  Gal.  (1894)  i,  212-219,  223-226 ;  pi.  xm, 
figs.  3,  4. 

4  G.  M.  1895,  124. 

6  Miss  Gardiner,  Q.  J.  G.  S.  (1890)  xlvi,  570-573. 


METAMORPHOSED   LIMESTONES.  305 

consist  of  quartz,  light  and  dark  micas,  the  former  predomin- 
ating, red  garnet,  and  subordinate  felspar.  The  mica  gives 
a  foliated  character  to  the  mass,  and  the  quartz  tends  to 
aggregate  in  little  knots  or  lenticles. 

Metamorphism  of  calcareous  rocks.  It  appears 
that,  under  the  conditions  which  rule  in  ordinary  cases  of 
metamorphism  by  heat,  carbonic  acid  is  not  driven  off  from 
lime-carbonate,  except  in  presence  of  available  silica  to  replace 
it.  Thus  a  pure  limestone  is  not  altered  in  chemical  com- 
position by  metamorphism.  It  is,  however,  at  a  sufficiently 
high  temperature,  recrystallized  into  a  fine  or  coarse-grained 
marble,  in  which  all  traces  of  clastic  and  organic  structures 
are  effaced.  This  is  seen  locally  in  the  Mountain  Limestone 
against  the  Whin  Sill  of  Teesdale,  in  the  purer  parts  of  the 
Coniston  Limestone  near  the  Shap  granite,  etc. 

Most  metamorphosed  limestones,  however,  have  had  suf- 
ficient impurities  to  give  rise  to  various  lime-bearing  silicates, 
which  are  found  in  the  recrystallized  limestone  as  crystals, 
crystalline  aggregates,  patches,  plumose  tufts,  etc.  The  chief 
characteristic  minerals  liave  been  noted  above.  Two  or  more 
of  them  often  occur  in  association,  and  sometimes  with  a 
regular  arrangement.  Thus  some  beds  of  the  Coniston  Lime- 
stone near  the  Shap  granite  enclose  large  crystals  of  idocrase 
in  stellate  groups  or  nests,  each  nest  surrounded  by  a  shell 
composed  largely  of  felspar.  Metamorphosed  limestones  in 
Glen  Derry,  near  the  Cairngorm  granite,  contain  aggregates  of 
garnet.  In  the  Glen  Tilt  rocks  we  find  chiefly  amphibole- 
minerals — tremolite,  actinolite,  and  green  or  even  brown 
hornblende.  A  band  of  crystalline  limestone  near  Tarfside 
in  the  highly  metamorphosed  area  of  Forfarshire  has  green 
hornblende,  zoisite,  felspar,  quartz,  sphene,  and  other  minerals. 
Fine  examples  of  the  production  of  lime- silicates  (wollastonite, 
scapolite,  felspars,  pyroxenes,  etc.}  are  furnished  by  the 
crystalline  limestones  bordering  the  gabbros  of  the  Adiron- 
dacks  and  the  Lake  Champlain  district1.  Crystalline  lime- 
stones with  accessory  minerals  of  metamorphic  origin  may 
attain  a  considerable  development  in  areas  of  'regional' 

1  Kemp,  Bull.  Geol.  Soe.  Amer.  (ia94)  v,  223;  (1895)  vi,  241-262; 
C.  H.  Smyth,  jr.,  ibid.  263-284. 

H.  P.  20 


306  LIME-SILICATE-ROCKS. 

metamorphism.     The  'cipollino'  of  the   Italian  geologists  is 
a  rock  of  this  kind  containing  mica  and  other  silicates. 

The  most  striking  effects,  however,  are  produced  in  very 
impure  limestones  or  in  calcareous  shales,  slates,  or  tuffs. 
In  these  the  carbonic  acid  is  completely  eliminated,  and  the 
whole  converted  into  a  lime-silicate-rock  (the  German  'Kalk- 
silikathornfels'  or  'Kalkhornfels').  It  appears  too  that  quite 
a  moderate  amount  of  calcareous  material  in  shales,  tuffs,  etc., 
suffices  to  make  the  metamorphism  take  this  line  instead  of 
those  described  under  the  head  of  argillaceous  rocks.  The 
metamorphosed  rocks  consist  of  aggregates,  usually  but  not 
always  fine-grained  and  compact,  of  silicates  rich  in  lime  with 
sometimes  quartz,  pyrites,  or  other  minerals.  Several  of  these 
minerals  occur  in  association,  giving  rise  to  rocks  of  complex 
constitution ;  and  beds  differing  slightly  in  the  amount  and 
nature  of  their  non-calcareous  material  result  in  different 
mineral-aggregates.  Numerous  types  are  illustrated  by  the 
metamorphosed  Coniston  Limestones  at  Wasdale  Head,  where 
they  abut  on  the  Shap  granite.  The  purer  beds,  as  already 
remarked,  are  converted  into  crystalline  limestones,  but  the 
calcareous  shales  and  tuffs  have  had  their  carbonate-minerals 
completely  destroyed.  The  Upper  Coniston  Limestone  is 
extensively  converted  into  a  compact  porcellanous-looking  rock, 
in  which  irregular  crystalline  patches  and  grains  of  pyroxenes 
and  other  lime-bearing  silicates  are  recognizable.  In  some 
specimens  wollastonite  predominates,  in  others  augite  (ompha- 
cite),  in  others  tremolite;  and  various  associations  of  these 
and  other  minerals  can  be  noted  in  thin  slices1.  Anorthite 
and  probably  other  felspars  are  present,  sometimes  in  irregular 
crystal-plates  or  patches  with  ophitic  habit,  sometimes  in 
minute  granules.  In  the  compact  rocks  are  sometimes  enclosed 
stellate  groups  of  large  crystals  (idocrase  or  augite),  each 
group  surrounded  by  a  shell  chiefly  of  plagioclase  crystals2. 
A  bed  in  the  Lower  Coniston  Limestone  is  converted  into 
a  mass  of  garnet  and  idocrase.  The  garnet  (grossularite)  is  in 
good  crystals  enclosing  pyroxene-granules  and  enclosed  by  the 


1  Q.  J.  G.  S.  (1891)  xlvii,  pi.  xn,  figs.  3,  4. 

2  Ibid.  (1893)  xlix,  pi.  xvn,  fig.  6. 


LIME-SILICATE-KOCKS. 


307 


clear  idocrase1  (fig.  72).  It  shews  the  optical  anomalies  noted 
above2.  A  considerable  variety  of  lime-silicate-rocks  is  found 
in  the  Cromdale  Hills,  etc.,  in  the  eastern  Highlands  of 
Scotland3. 


FIG.  72.     GARNET-IDOCRASE-ROCK  (METAMORPHOSED  CONISTON  LIME- 
STONE),   NEAR    SHAP   GRANITE,    WASDALE    HEAD,    WESTMORLAND;       X  20. 

The  highly  refringent  crystals  are  the  lime-garnet  (grossularite)  and 
the  clear  mineral  forming  the  matrix  is  idocrase.  Both  enclose  abundant 
pyroxene-granules  [2730]. 

More  remarkable  effects  are  produced  when  there  has 
been  an  introduction  of  boric  acid  into  the  rock  during  the 
metamorphism.  At  South  Brent,  on  the  border  of  the  Dart- 
moor granite,  Busz  has  remarked  a  Devonian  limestone 
converted  into  an  aggregate  of  birefringent  garnet  and 
interstitial  datolite.  Axinite  is  another  mineral  occurring 
in  like  connexion. 

Of  special  interest  is  the  dedokmitization  of  dolomite- 
rocks  by  metamorphism.  Here  the  dolomite  is  reduced  to 
calcite,  while  its  magnesia  enters  into  new  minerals.  One 

1  Q.  J.  G.  S.  (1891)  xlvii,  pi.  xii,  fig.  1. 

2  Ibid.  p.  312. 

3  Teall,  Mem.  Geol.  Sur.  Scot.,  Expl.  Sheet  75  (1896),  36,  44. 

20—2 


308 


DEDOLOMITIZATION. 


well-marked  type  arising  in  this  way  consists  of  calcite  and 
forsterite  (fig.  73),  and  such  a  rock  may  be  converted  into  a 


FlG.    73.      FOBSTERITE-MABBLE    (METAMORPHOSED    CAMBRIAN   DOLOMITE), 
NEAR   GRANITE,    KlLCHRIST,    SKYE  ;       X  20. 

Shewing  crystals  of  olivine  (forsterite)  in  a  calcite-mosaic  [2398]. 

serpentinous  marble  or  'ophicalcite.'  Even  a  pure  dolomite- 
rock,  free  from  siliceous  or  other  impurity,  may  be  dedolo- 
mitized ;  and  in  this  way  have  been  formed  the  rocks  known 
in  the  Tirol  as  'predazzite'  and  'pencatite,'  which  are  granular 
aggregates  of  calcite  and  brucite,  the  latter  probably  arising 
from  the  hydration  of  periclase.  These  and  other  types  are 
found  among  the  metamorphosed  equivalents  of  the  Cambrian 
dolomite-rocks  at  Ledbeg  in  Sutherland,  on  the  border  of  the 
Loch  Borolan  intrusion1,  and  also  in  Skye,  where  the  same 
group  of  strata  is  highly  metamorphosed  by  the  Tertiary 
granite  and  gabbro. 

Metamorphism  of  igneous  rocks.  Although  the 
thermal  metamorphism  of  plutonic  rocks,  lavas,  volcanic  ashes, 
etc.,  has  not  yet  received  very  much  attention,  it  offers  many 
points  of  interest  and  importance.  Many  of  these  features 

1  Teall,  Summary  of  Progress  Geol  Sur.  for  1900,  153,  154. 


METAMORPHOSED   RHYOLITES   AND   TUFFS.  309 

are  exhibited  by  the  Ordovician  volcanic  series  of  the  Lake 
District  in  the  neighbourhood  of  the  granite  intrusions  of 
Shap  and  Eskdale. 

The  acid  igneous  rocks  are  much  less  susceptible  to  thermal 
metamorphism  than  those  of  intermediate  and  basic  composi- 
tion. The  rhyolites  near  the  Shap  granite  do  not,  as  a  rule, 
shew  any  changes  that  can  be  clearly  attributed  to  the  effects 
of  heat.  Where,  however,  decomposition-products  existed  in 
the  original  rocks,  they  have  given  rise  to  metamorphic  minerals. 
In  particular,  the  green  pinitoid  substance  is  converted  into  a 
mixture  of  white  and  brown  micas.  The  coarsely  spheroidal 
('nodular')  rhyolites  illustrate  this  point.  The  spheroids  had, 
prior  to  metamorphism,  been  altered  in  the  usual  fashion  into 
complex  nodules  having  concentric  shells  of  rhyolite  substance 
and  of  weathering-products.  In  the  metamorphosed  nodules 
the  shells  of  unweathered  rhyolite  remain  unaltered,  the  flinty 
siliceous  zones  are  converted  into  quartz-mosaic  with  a  little 
mica,  and  the  pinitoid  substance  is  changed  into  biotite  and 
muscovite.  In  the  cracks  which  divided  the  shells  there  may 
be  a  little  blue  tourmaline. 

The  fragmental  rocks  associated  with  these  rhyolites 
were  of  much  less  acid  composition,  and  were  probably  more 
weathered  prior  to  the  metamorphism.  Hence  they  shew 
more  change,  the  production  of  biotite  being  often  observed. 
As  in  argillaceous  rocks,  little  spots  relatively  clear  of  mica 
are  sometimes  present :  these  shew  a  crystalline  reaction  and 
may  be  andalusite.  The  spots  disappear  with  more  complete 
metamorphism,  but  crystals  or  grains  of  andalusite  or  cyanite 
are  sparingly  developed,  and  finally  the  rock  is  completely 
recrystallized  into  a  finely  granular  mosaic  with  a  certain 
amount  of  biotite,  a  little  opaque  iron-ore,  etc.  Relatively 
large  crystals  of  felspar  enclosed  in  the  tuffs  are  replaced  by  a 
new  felspar-mosaic,  only  the  general  outline  of  the  original 
crystal  being  preserved. 

In  the  intermediate  and  basic  rocks  metamorphism  may 
give  rise  to  important,  changes.  Diorites  are  metamorphosed 
in  the  Malvern  range,  the  results,  however,  being  complicated 
by  dynamic  changes.  As  described  by  Dr  Callaway1,  the  chief 

1  Q.  J.  G.  S.  (1889)  xlv,  485,  etc. 


310          METAMORPHOSED   GABBROS    AND   DIABASES. 

effect  clearly  referable  to  heat  is  the  replacement  of  horn- 
blende by  a  deep  brown  biotite  in  the  vicinity  of  an  intruded 
granite1.  It  appears  that  the  hornblende  had  been,  at  least 
to  some  extent,  previously  converted  into  a  chloritic  mineral. 
The  plagioclase  is  stated  to  give  rise  to  white  mica.  The  same 
author2  describes  the  metamorphism  of  diorite  by  a  granitic 
intrusion  at  Gal  way  Bay,  where  recrystallized  plagioclase  is 
observed,  and  the  hornblende  has  given  place  to  a  chloritic 
mineral,  epidote,  and  rarely  biotite. 

The  Carrock  Fell  granophyre,  in  Cumberland,  has  produced 
metamorphism  in  a  very  basic  type  of  gabbro.  In  some  ex- 
amples the  apatite  and  iron -ores  are  unchanged,  the  turbid 
felspars  become  clear,  and  the  augite  is  converted  into  green 
actinolitic  hornblende  or  into  biotite.  The  latter  occurs  chiefly 
near  the  grains  of  iron-ores,  from  which  it  has  probably  taken 
up  some  ferrous  oxide  and  titanic  acid3.  In  other  specimens 
the  gabbro  shews  more  complex  changes. 

The  metamorphism  of  diabases  by  granitic  intrusions  has 
been  noticed  by  Allport4  in  Cornwall,  by  Lessen  in  the  Harz, 
etc.  Specimens  from  these  districts  shew  in  various  stages  the 
conversion  of  augite  into  hornblende  and  the  recrystallization 
of  the  felspar.  The  hornblende  produced  is  mostly  green,  but 
in  the  neighbourhood  of  the  iron-ores  (ilmenite)  it  is  sometimes 
brown.  Brown  mica  or  scaly  patches  of  chlorite  may  be  found 
instead  of  hornblende,  and  these  often  give  indications  of  being 
formed  not  directly  from  augite  but  from  its  decomposition- 
products.  In  the  Isle  of  Skye  similar  effects  are  to  be  observed 
in  diabase  dykes  cut  off  and  metamorphosed  by  the  granite  of 
Beinn  an  Dubhaich  (fig.  74). 

The  augite-andesites  on  the  west  side  of  the  Shap  granite 
afford  fine  examples  of  thermal  metamorphism.  They  had 
undergone  considerable  change  prior  to  the  post-Silurian 

1  On  production   of  a  red  mica  in  a  diorite,  see  also  McMahon, 
Q.  J.  G.  8.  (1894)  1,  351. 

2  L.c.  p.  495. 

3  Q.  J.  G.  S.  (1894)  1,  pi.  xvii,  fig.  4.     See  also  Sollas  on  Carlingford 
district,  Trans.  Roy.  Ir.  Acad.  xxx,  493-496,  pi.  xxvi,  fig.  8,  xxvn,  figs. 
10-16. 

4  Q.  J.  G.  S.  (1876)  xxxii,  407-427.     For  figs,  see  Teall,  pi.  xvn,  and 
xxi,  fig.  2. 


METAMOKPHOSED  AUGITE-ANDESITES.  311 

intrusion  of  the  granite.     Chloritic  minerals,  calcite,  chalce- 
dony, and  quartz  had  been  formed  from  the  pyroxene  and 


FIG.  74.     METAMORPHOSED  DIABASE  DYKE,  CLOSE  TO  GRANITE, 
KILCHRIST,  SKYE;     x  20. 

The  augite  is  totally  transformed  to  a  pale,  rather  fibrous  hornblende, 
except  round  the  granules  and  skeletons  of  iron-ore,  where  its  place  is 
taken  by  biotite.  The  felspar  crystals  have  become  quite  clear,  but 
narrow  chloritic  veins  traversing  them  have  been  converted  to  hornblende 
[3207]. 

felspar,  and  were  partly  disseminated  through  the  rock,  but 
especially  collected  in  little  veins  and  in  the  vesicles.  These 
alteration-products  were  the  elements  most  readily  affected 
by  heat.  The  chloritic  mineral  has  been  converted  into 
biotite,  or,  where  it  was  associated  with  calcite,  into  green 
hornblende  (notably  in  the  vesicles)  :  chalcedonic  silica  has 
been  transformed  into  crystalline  quartz1.  The  rocks  are  more 
altered  nearer  the  granite,  and  new  minerals  appear,  such  as  a 
purplish-brown  sphene,  magnetite,  and  pyrites  ;  the  plagioclase 
phenocrysts  are  replaced  by  a  mosaic  of  new  felspar  substance  ; 
and  finally  the  whole  mass  of  the  rock  is  found  to  be  reconsti- 
tuted, the  ground  becoming  a  fine-textured  mosaic  of  clear 

1  Q.  J.  G.  S.  (1891)  xlvii,  294-298 ;  pi.  xi,  figs.  4,  5. 


312  METAMORPHOSED   BASALTS. 

granules.     Mr  Kynaston1  has  described  similar  effects  in  the 
Old  Red  Sandstone  andesites  bordering  the  Cheviot  granite. 

A  more  basic  type  of  lava,  on  the  north  side  of  the  Shap 
granite,  shews  phenomena  on  the  whole  very  similar  to  the 
preceding  ;  but,  owing  to  the  larger  percentage  of  lime  present, 
the  minerals  produced  are  in  part  different.  Green  hornblende 
predominates  over  biotite  among  the  coloured  constituents  of 
the  metamorphosed  rocks,  and  an  augite,  colourless  in  slices, 
is  also  formed,  especially  in  veins  and  amygdules.  Epidote 
is  another  characteristic  mineral,  and  sphene,  pyrites,  and 
magnetite  occur  as  before.  Especially  noteworthy  is  the  form- 
ation of  numerous  lime-bearing  silicates  from  the  contents  of 
the  vesicles :  grossularite  occurs,  as  well  as  hornblende  and 
actinolite,  epidote,  augite,  and  quartz.  In  the  centre  of  the 
largest  amygdules  some  residual  calcite  is  found,  recrystallized 
but  not  decomposed2.  A  basic  hypersthene-bearing  lava  (the 
Eycott  type)  is  metamorphosed  by  the  Carrock  Fell  gabbro3, 
the  bastite  pseudomorphs  after  hypersthene  being  converted 
into  a  pale  hornblende.  Here  the  transformation  of  the  rocks 
is  not  always  complete,  the  large  labradorite  phenocrysts  being, 
as  a  rule,  not  recrystallized  into  a  mosaic,  but  only  cleared  of 
their  dusty  inclusions  (fig.  75).  The  metamorphosed  Ordovi- 
cian  lavas  near  the  Galloway  granites*  recall  in  many  respects 
the  Shap  rocks.  A  lime-garnet  is  frequently  met  with,  and 
new  felspar  occurs  both  in  the  body  of  the  rock  and  in  the 
amygdules. 

The  Tertiary  basaltic  lavas  of  Skye  are  often  considerably 
metamorphosed  by  the  later  intrusions  of  gabbro  and  granp- 
phyre.  One  interesting  result  is  the  formation  of  felspar  in 
the  amygdules5.  It  is  produced,  together  with  epidote,  zoisite, 
actinolite,  etc.,  mainly  at  the  expense  of  soda-lime-zeolites. 
In  the  mass  of  the  rock  the  chief  change  is  usually  the 
conversion  of  the  augite  to  greenish  fibrous  hornblende.  In 
the  highest  grade  of  metamorphism,  however,  hornblende  is 

1  Tr.  Edin.  G.  S.  (1901)  viii,  18-26. 

2  Q.  J.  G.  S.  (1893)  xlix,  360-364,  pi.  xvn,  figs.  1-4. 
»  Ibid.  (1894)  1,  332. 

4  Teall,  Ann.  Rep.  Geol.  Sur.  for  1896,  47  ;  Mem.  Geol.  Sur.,  Silur. 
Eocks  Scot.  (1899)  647-650. 

5  Q.  J.  G.  S.  (1896)  lii,  386,  387. 


METAMORPHOSED  BASIC  TUFFS.  313 

not  produced,  augite  being  found  both  in  the  body  of  the  rock 
(recrystallized  in  common  with  the  felspar)  and  in  the 
amygdules  (associated  with  new  felspar  which  replaces 
zeolites). 


Ib 


FIG.  75.    METAMORPHOSED  BASIC  LAVA  ENCLOSED  IN  THE 

GABBBO  OF  CABROCK  FELL,  CUMBEBLAND  ;  X  20. 

The  rock  was  originally  a  hypersthene-basalt  belonging  to  the  Eycott 
Hill  group  (see  fig.  46).  The  porphyritic  felspars  have  become  clearer  (Ib), 
their  large  inclusions  disappearing ;  the  pyroxenes  or  their  weathering- 
products  have  been  converted  chiefly  into  a  pale  hornblende  (hb)  or 
locally  into  biotite  (bi) ;  the  magnetite  has  recrystallized  in  good 
octahedra ;  and  the  felspars  of  the  ground-mass  are  now  a  clear 
aggregate,  which  appears  almost  homogeneous  in  natural  light  [1550]. 

The  tuffs  of  basic  and  intermediate  character  near  the 
Shap  granite  have  much  resemblance  to  the  lavas  as  regards 
their  metamorphism.  Brown  mica  is  the  usual  ferro-magnesian 
mineral  formed,  amphibole  being  less  common.  Magnetite  is 
never  abundant,  and  sphene  is  wanting.  The  most  metamorph- 
osed rocks  are  completely  reconstituted  into  a  very  fine- 
textured  aggregate  of  clear  granules,  in  which  lie  flakes  of 
biotite  parallel  to  either  original  lamination  or  cleavage, 
producing  a  kind  of  mica-schist.  Felspar  crystals  enclosed 
in  the  tuffs  are  either  transformed  into  pseudomorphs  of 
epidote  or  recrystallized  into  a  mosaic1. 

1  Q.  J.  G.  S.  (1893)  xlix,  pi.  xvn,  fig.  5. 


314  METAMORPHOSED   CRYSTALLINE   SCHISTS. 

Metamorphism  in  crystalline  schists,  etc.  On  this 
subject  there  is  not  a  large  amount  of  information,  and  it 
appears  that  crystalline  schists  of  various  kinds  are,  as  a  whole, 
less  susceptible  to  thermal  changes  than  sedimentary  rocks. 
The  metamorphism  of  phyllites  and  mica-schists  has  been 
studied  in  the  Adamello  range,  in  the  Riesengebirge,  in  New 
Hampshire1,  on  the  Hudson  River2,  etc.  In  some  respects  the 
phenomena  resemble  those  seen  in  argillaceous  strata,  the 
production  of  biotite,  andalusite,  etc.,  being  characteristic ; 
but  there  are  sometimes  quite  special  peculiarities,  in  particular 
the  formation  of  minerals  very  rich  in  alumina.  Cordierite 
is  sometimes  extremely  abundant,  while  pleonaste  and  other 
spinels  and  pure  corundum  are  noted  in  several  localities. 

In  the  southern  Highlands  of  Scotland  Mr  dough  has 
observed  the  crystalline  schists  to  be  metamorphosed  by  the 
granitic  intrusions  of  the  Garabal  Hill  district.  Within  a 
mile  of  the  junction  the  albite-schists  begin  to  develop  small 
prisms  of  andalusite,  which  increase  in  size  and  abundance, 
and  at  the  same  time  nests  of  dark  mica  become  plentiful. 

1  Hawes,  A.  J.  S.  (1881)  xxi,  21-32. 

2  G.  H.  Williams,  ibid.  (1888)  xxxvi,  254-266. 


CHAPTER  XXI. 

DYNAMIC  METAMORPHISM. 

IN  this  chapter  will  be  noticed  some  of  the  effects,  mineral- 
ogical  and  structural,  produced  in  rock- masses  by  the  operation 
of  great  mechanical  forces.  Among  the  mineralogical  changes 
we  ought  logically  to  separate  those  due  to  pressure  from  those 
due  to  mechanically  generated  heat,  the  latter  belonging  rather 
to  the  preceding  chapter.  This  distinction  we  shall  make,  so 
far  as  our  actual  knowledge  goes. 

The  consideration  of  dynamic  metamorphism  in  comparat- 
ively yielding  rock-masses  has  already  been  partly  anticipated 
in  the  chapter  devoted  to  argillaceous  sediments  :  phenomena 
more  striking,  or  at  least  more  easily  investigated,  are  now  to 
be  noticed  in  crystalline  and  other  rocks  of  more  stubborn 
consistency. 

Strain-phenomena  in  crystalline  rocks.  A  frequent 
effect  of  strain  in  the  component  crystals  of  a  stubborn  rock- 
mass  is  a  modification  of  the  optical  properties,  which  at  once 
becomes  apparent  between  crossed  nicols.  Instead  of  being 
dark  throughout  for  certain  definite  positions,  a  crystal  shews 
dark  shadows  which  move  across  it  as  the  stage  is  rotated,  owing 
to  the  directions  of  extinction  varying  from  point  to  point. 
These  strain-shadows1  are  best  seen  in  quartz,  and  are  very 
common  in  the  granitic  and  gneissic  rocks,  quartzites,  etc.,  of 
countries  like  the  Scottish  Highlands  or  the  older  parts  of 

1  Mr  Blake  styles  this  appearance  '  spectral  polarization.'  It  is 
spoken  of  by  some  foreign  writers  as  '  undulose  extinction.' 


316 


STRAIN-PHENOMENA   IN    CRYSTALS. 


Norway,  which  have  been  the  theatre  of  great  crust- movements. 
Again,  a  mineral  such  as  garnet,  normally  isotropic,  may 
become  birefringent  (e.g.,  in  the  Eddystone  gneiss). 

Flexible  minerals,  such  as  micas,  often  shew  bending  of 
their  crystals,  or,  again,  they  yield  by  a  shearing  movement 
analogous  to  lamellar  twinning  parallel  to  definite  directions 
known  as  gliding-planes  (Ger.  Gleitflachen).  In  some  minerals, 
such  as  the  plagioclase  felspars,  the  gliding-planes  coincide 


FIG.  76.     SECONDARY  TWIN-LAMELLATION  IN  PLAGIOCLASE  FELSPAR,  DUE  TO 

STRAIN,  IN  GABBRO,  ILGERSHEIM,  NAHE  DISTRICT  ;     X  20,  CROSSED  NICOLS. 

In  places  where  the  strain  has  been  greatest  the  crystals  have  yielded 
along  cracks.  The  mineral  at  the  top  of  the  figure  is  diallage  converted 
into  hornblende  [1408]. 

with  natural  twin-planes1,  and  the  secondary  twinning  can  be 
distinguished  from  original  lamellation  only  by  its  inconstant 
character  and  its  relation  to  bending  or  other  strain-phenomena. 
It  is  very  clearly  seen  in  such  rocks  as  the  norites  of  Hittero, 
Seiland,  and  Bekkafjord  in  Norway2,  where  the  original  twin- 
lamellae  of  the  felspars  are  rather  broad.  Sometimes,  in  one 
crystal,  the  closeness  of  the  secondary  lamellae  is  seen  to  increase 

1  Judd,  Q.  J.  G.  S.  (1885)  xli,  363-366,  pi.  x,  fig.  1. 

2  Cohen  (3),  pi.  LXXIX,  figs.  1,  2. 


STRAIN-PHENOMENA   IN   CRYSTALS.  317 

with  the  strain,  until  the  crystal  has  yielded  along  a  crack  or 
a  granulated  vein  (fig.  76).  In  some  rocks  there  seems  to  be 
evidence  of  the  microcline-structure  being  set  up  in  orthoclase 
as  a  result  of  strain. 

Quartz  sometimes  shews  rows  of  fluid-pores  marking  direc- 
tions of  shearing-strain,  and  parallel  to  actual  planes  of  faulting 
if  the  crystal  has  yielded1.  The  lines  of  pores  can  be  traced 
through  contiguous  crystal-grains ;  or  entering  another  mineral, 
such  as  felspar,  they  may  become  actual  planes  of  discon- 
tinuity. 

It  appears  that  the  schiller-structures2,  so  characteristic 
of  certain  minerals  in  deep-seated  rocks,  may  also  be  produced 
as  secondary  phenomena  by  pressure.  A  typical  structure  is 
that  in  which  cavities  of  definite  form  and  orientation  ('negative 
crystals')  are  developed  along  certain  planes,  and  filled,  or 
partially  filled,  by  material  dissolved  out  from  the  enclosing 
crystal.  Hypersthene  affords  a  good  example.  The  *  solution- 
planes  '  (Ger.  Losungsflachen)  proper  to  a  mineral  are  parallel 
to  one  or  more  crystallographic  planes  ;  but  after  a  secondary 
lamellar  twinning  has  been  set  up  in  a  crystal,  the  gliding- 
planes  become  the  easiest  solution-planes.  Pyroxenes,  felspars, 
and  olivine  are  minerals  often  affected  by  schiller-structures. 

Crystals  of  brittle  minerals  subjected  to  stress  have  often 
yielded  by  actual  cracks,  which  may  have  a  definite  direction 
throughout  the  rock,  being  perpendicular  to  the  maximum 
tension,  and  so  parallel  to  the  maximum  pressure.  This  is 
sometimes  seen  in  quartz  and  felspars,  but  most  commonly  in 
the  garnet  of  granulites,  eclogites,  gneisses,  and  crystalline 
schists.  As  a  further  stage,  the  portions  of  a  fractured  crystal 
may  be  separated  and  rolled  over,  or  drawn  out  in  the  direc- 
tion of  stretching  or  flowing  movement  in  the  solid  rock.  It 
is  noticeable  that  quartz  shews  these  phenomena  much  oftener 
than  felspar :  the  former  mineral,  though  harder  than  the 
latter,  is  more  brittle. 


1  Judd,  M.  M.  (1886)  vii,  82,  pi.  m,  fig.  1. 

2  Judd,  Q.  J.  G.  S.  (1885)  xli,  374-389,  pi.  x-xn ;  M.  M.  (1886)  vii, 
81-92,  pi.  m. 


318 


CRUSH-PHENOMENA   IN   ROCKS. 


Cataclastic  structures.  The  phenomena  of  internal 
fracture  and  crushing  of  hard  rocks  ('  cataclastic '  structures  of 
Kjerulf)  are  to  be  seen  in  endless  variety  in  some  regions  of 
great  mechanical  disturbance.  They  may  be  developed  in  less 
or  greater  degree  ;  they  may  affect  some  or  all  of  the  mineral 
constituents  of  a  composite  rock ;  they  may  or  may  not  tend 
to  a  parallel  arrangement  of  the  elements.  In  one  type  the 
rock-mass  breaks  up  along  definite  surfaces  of  sliding,  the 
material  bordering  the  cracks  being  often  ground  down  by 
friction  :  this  is  brecciation  in  situ.  The  irregularly  intersecting 
surfaces  divide  the  rock  into  angular  fragments  ;  but  these 
may  be  rolled  over  and  their  angles  rubbed  [off,  so  that  a 
'friction-conglomerate'  as  well  as  a  'friction-breccia'  may 
arise,  especially  along  faults  and  thrust-faults  (e.g.,  Lake 
District).  According  as  the  new  structure  is  on  a  large  or  a 
small  scale,  the  fragments  may  be  recognizable  pieces  of  rocks 
or  portions  of  constituent  crystals  of  an  originally  crystalline 
rock. 

Again,  we  sometimes  find  the  larger  elements  of  a  rock — 
grains  of  quartz,  crystals  of  felspar,  etc. — surrounded  by  a 
border  of  finely  granular  material  furnished  by  the  grinding 
down  of  the  crystal  itself  and  adjacent  ones.  This  is  the 
mwter-structure  (Ger.  Mortelstructur)  of  Tornebohm.  As  a 
further  stage,  the  finely  granular  portion  of  the  rock  may 
make  up  the  chief  part  of  its  bulk,  forming  a  matrix  which 
encloses  portions  of  crystals  not  yet  destroyed  but  indicating 
by  irregular  polarization  their  strained  condition.  Beautiful 
examples  are  seen  among  the  crushed  quartzites  and  gneisses 
of  Sutherland  (fig.  77). 

In  many  cases  mechanical  forces  having  a  definite  direction 
have  caused  uncrushed  fragments  to  assume  an  eye-shaped  or 
lenticular  form  (Ger.  Augenstructur)  with  their  long  axes 
perpendicular  to  the  maximum  pressure,  and  so  parallel  to  one 
another  and  to  any  schistose  structure  in  the  matrix  (fig.  80,  A). 
In  such  cases  the  crushed  matrix  usually  has  a  more  or  less 
well-marked  parallel  structure  or  schistosity,  in  part  analogous 
to  slaty  cleavage.  The  final  result  of  the  grinding  down  and 
rolling  out  processes  is  the  type  of  rock  named  mylonite  by 


CRUSH-PHENOMENA   IN   ROCKS. 


319 


Professor  Lap  worth1,  in  which,  except  perhaps  for  occasional 
uncrushed  'eyes,'  all  original  structures  are  lost.  In  these 
much  crushed  rocks  the  'eyes'  no  doubt  represent  in  many 
cases  porphyritic  crystals,  usually  of  felspar,  in  what  was  once 


FIG.  77.    ADVANCED  CATACLASTIC  STRUCTURE  IN  GNEISS,  SOUTH 
SLOPE  OF  BEINN  MOR  OF  ASSYNT,  SUTHERLAND  ;    x  20. 

The  greater  part  of  the  rock  is  completely  broken  down,  and  has 
partly  taken  on  the  parallel  structure  of  a  mylonite.  A  large  grain  of 
quartz  is  only  partly  crushed,  and  this  between  crossed  nicols  shews 
strain-shadows  [1641]. 

an  ordinary  igneous  rock.  It  is  evident,  however,  that,  in  the 
absence  of  such  indications,  it  must  often  be  impossible  to 
determine  by  microscopical  study  alone  the  nature  of  a  rock 
whose  original  structures  have  been  totally  obliterated. 

Mineralogical  transformations,  etc.  In  extreme 
stages  of  crushing  of  crystalline  rocks,  the  changes  produced 
are  by  no  means  purely  mechanical.  In  consequence  of  the 
stress  and  subsequent  relief  a  recrystallizatian  of  minerals 
may  be  effected,  resulting  in  the  clear,  finely  granular  aggregate 
which  forms  a  large  part  of  some  dynamo -metamorphic  rocks2. 

1  See  Page  (Lapworth),  Introd.   Text-book  Geol.  12th  ed.,  figs,  on 
p.  107. 

2  Cf.  Teall,  p.  175,  figures. 


320       MINERALOGICAL   CHANGES   UNDER   PRESSURE. 

It  must  be  remembered,  however,  that  thermal  metamorphism 
due  to  mechanically  generated  heat  may  complicate  the 
strictly  dynamic  changes. 

Further,  atomic  as  well  as  molecular  rearrangement  has 
operated  in  greater  or  less  degree  in  any  dynamo-metamorphic 
rock  not  of  the  simplest  constitution.  Certain  mineralogical 
transformations  seem  to  be  characteristic  of  dynamical  meta- 
morphism, being  either  developed  by  the  action  of  great  pressure 
or  at  least  facilitated  by  pressure  even  when  they  can  also 
take  place  without  that  condition1.  It  should  be  noticed  that 
in  crystalline,  and  generally  in  hard,  rocks,  these  mineralogical 
changes  begin  before  any  important  structural  modifications 
are  produced. 

One  characteristic  change  is  the  production  of  colourless 
mica  at  the  expense  of  alkali-felspars.  The  mineral  may  be 
formed  at  the  margin  of  a  crystal  squeezed  against  its  neigh- 
bours or  on  surfaces  of  lamination  or  of  movement  in  a  fel- 
spathic  rock  :  in  such  cases  it  takes  the  filmy  form  known  as 
sericite.  Or  it  may  replace  the  interior  of  a  crystal  partially 
or  almost  wholly.  Potash-felspar  gives  rise  to  muscovite, 
soda-felspar  to  paragonite. 

A  characteristic  alteration  in  the  soda-lime-felspars  results 
in  the  minutely  granular  aggregate  which  has  been  called 
'  saussurite,'  and  is  not  always  of  precisely  the  same  nature2. 
The  soda-bearing  silicate  of  the  felspar  separates  out  as  very 
minute  clear  crystals  of  albite,  while  the  lime-bearing  silicate, 
in  conjunction  with  other  constituents  of  the  rock,  goes  to  form 
minerals  rich  in  lime.  Zoisite  is  a  characteristic  mineral,  or 
its  place  may  be  taken  by  yellow  or  colourless  epidote ;  and 
needles  of  actinolite  may  also  occur.  (Compare  fig.  78.) 

The  conversion  of  plagioclase  into  scapolite  under  dynamic 
action  seems  to  be  a  more  complex  process,  involving  the 
presence  of  sodium  chloride  in  solution3. 

1  See  G.  H.  Williams,  Bull.  62  U.  S.  Geol.  Surv.  (1890)  Ch.  i. 

2  Teall,  149-152.     For  a  somewhat  similar  process  of  '  granulation ' 
of  plagioclase  resulting  in  a  fine  mosaic  of  albite,  etc.,  see  Hyland,  G.  M. 
1890,  205-208.     Cf.  Williams,  I.e.  58-60,  68,  69,  figures. 

3  Judd,  M.  M.  (1889)  viii,  186-198,  pi.  ix. 


MINERALOGICAL   CHANGES   UNDER   PRESSURE. 


321 


Other  changes  common  in  dynamic  metamorphism  are  the 
conversion  of  olivine  into  tremolite  or  anthophyllite  and  talc, 


ca, 


cut 


FIG.  78.     SAUSSURITE-QABBRO,  NORWEGIAN  BOULDER  ON 
THE  YORKSHIRE  COAST  ;     x  20. 

The  portion  figured  consists  of  patches  of  pale  greenish  fibrous 
hornblende  or  actinolite  (at),  calcite  (ca),  and  chlorite,  prisms  of  zoisite 
(z),  grains  of  epidote  (ep),  and  little  clear  crystals  of  secondary  felspar. 
The  so-called  '  saussurite '  is  a  similar  aggregate  on  a  more  minute  scale 
[1049]. 

and  the  production  of  granular  sphene  at  the  expense  of 
ilmenite  or  other  titaniferous  minerals.  Augite  gives  rise 
when  crushed  to  chlorite.  The  conversion  of  augite  or  other 
pyroxenes  into  green  hornblende  is  also  a  common  feature  in 
regions  of  dynamic  metamorphism  :  perhaps  this  is  one  of  the 
transformations  that  should  be  ascribed  to  the  heat  generated 
in  the  crushing.  It  is  a  very  wide-spread  phenomenon1. 

The  borders  ('  reaction-rims ')  sometimes  noticed  at  the 
junction  of  two  different  minerals  in  a  crystalline  rock  have 
in  many  cases  been  attributed  to  dynamic  metamorphism. 
(See  above,  p.  77.) 

1  See,  e.g.,  R.  D.  Irving,  A.  J.  S.  (1883)  xxvi,  27-32  ;  G.  H.  Williams, 
ibid.  (1884)  xxviii,  259-268 ;  Teall,  Q.  J.  G.  S.  (1885)  xli,  133-144. 


H.  P. 


21 


322          SOME    AREAS   OF   DYNAMIC    METAMORPHISM. 

Illustrative  examples.  After  the  above  remarks  it 
will  be  sufficient  to  mention  a  few  cases  in  illustration  of  what 
is  a  very  wide  and  only  partly  explored  field  of  research. 
Much  valuable  information  has  been  published  by  observers 
in  various  European  districts,  and  especially  by  Lehmann  in 
his  work  on  the  Saxon  Granulite  Mountains,  with  numerous 
photographic  plates'.  The  most  complete  study  in  English  of 
a  region  of  dynamic  metamorphism  is  perhaps  that  by  G.  H. 
Williams  of  the  '  greenstone-schists/  etc.,  of  the  Lake  Superior 
region,  which  further  contains  a  general  summary  of  know- 
ledge on  the  subject2.  The  dominant  types  of  rocks  in  the 
areas  there  studied  have  been  basic  eruptives,  probably  true 
lavas  in  great  part,  and  these  are  now  represented  by  chlorite- 
and  hornblende-schists.  Gabbros,  diorites,  granites,  and 
quartz-porphyries  have  also  been  included,  and  shew  their 
appropriate  types  of  alteration.  The  author  traces  in  detail 
the  processes  of  uralitization,  chloritization,  epidotization, 
saussuritization,  sericitization,  etc.,  as  well  as  the  structural 
changes  undergone  by  the  rocks. 

In  our  own  country,  and  especially  in  some  parts  of  the 
Scottish  Highlands,  the  phenomena  of  dynamic  metamorphism 
are  exhibited  on  an  extensive  scale3.  Dykes  in  the  western 
part  of  Sutherland  shew  very  clearly  the  conversion  of  diabase 
into  hornblende-schist,  and  an  instance  of  this  has  been 
described  in  detail  by  Mr  Teall4.  The  augite  is  transformed 
into  green  hornblende,  and  the  felspar  has  recrystallized  in 
water-clear  grains,  while  the  titaniferous  iron -ore  has  also 
been  altered,  giving  rise  frequently  to  granular  sphene. 
These  mineralogical  changes  may  be  produced  without  any 
schistose  structure,  but  the  massive  hornblendic  rock  further 
becomes  in  places  a  typical  hornblende-schist.  This  is  at 
Scourie  :  other  examples  are  seen  near  Unapool,  on  Loch 

1  Entstehung    der    Altkrystallinischen    Schiefergesteine,    etc.      Bonn 
(1884),  Atlas. 

2  The  Greenstone  Schist  Areas  of  the  Menominee  and  Marquette  Regions 
of  Michigan,  Bull  62  U.  S.  Geol.  Surv.  (1890)  Ch.  i,  vi,  figures  and 
plates. 

3  See  Report  in  Q.  J.  G.  S.  (1888)  xliv,  429-435. 

*  Q.  J.  G.  S.  (1885)  xli,  133-144,  pi.  n ;  Brit.  Petr.  pi.  xix,  xx,  pp. 
197-200, 


DYNAMIC   METAMORPHISM   IN   SUTHERLAND. 


323 


Glencoul,  and  near  Loch  Assynt  (fig.  79).  At  Lochinyer 
dykes  of  enstatite-peridotite  pass  into  an  anthophyllite-schist, 
consisting  of  matted  aggregates  of  anthophyllite  prisms  or 


FIG.  79.     AMPHIBOLITE  OB  HOUNBLENDE-SCHIST,  FROM  THE  META- 
MORPHISM  OF   A   DIABASE    DYKE,    LOCH    ASSYNT,    SUTHERLAND  J       X  20. 

The  rock  now  consists  essentially  of  idiomorphic  hornblende  and 
clear  secondary  felspar,  with  some  magnetite.  The  slice  is  qut  parallel 
to  the  schistosity,  which  therefore  is  not  apparent  in  the  figure  [1664]. 

needles  with  little  patches  of  brilliantly  polarizing  talc  and 
large  rhombs  of  carbonates1. 

Near  Loch  Assynt  and  in  other  places  the  Lewisian  gneiss 
is  traversed  by  zones  of  crushing,  within  which  the  rock  is 
completely  reconstituted,  and  from  the  granitoid  assumes  the 
'granulitic'  structure.  The  rock  so  metamorphosed  shews  a 
rather  fine-textured  mosaic  of  clear  quartz  and  felspar,  enclos- 
ing imperfect  crystals  of  green  hornblende  and  ragged  flakes 
of  brown  mica  instead  of  the  original  pyroxene.  There  is  a 
marked  parallel  structure  and  some  tendency  in  the  several 
minerals  to  collect  into  little  lenticular  aggregates.  The  basic 

1  Mr  Teall  speaks  of  one  of  these  rocks  as  a  talc-gedrite-siderite-schist. 
In  other  examples  the  amphibole  mineral  is  a  monoclinic  one  (tremolite). 

21—2 


324  CHLORITE-SCHISTS,    ETC. 

and  ultrabasic  dykes  involved  in  these  crush-zones  are  meta- 
morphosed in  the  manner  just  described. 

Some  of  the  above-mentioned  changes  are  perhaps  to  be 
ascribed  rather  to  the  effects  of  mechanically  generated  heat 
than  to  pure  dynamic  metamorphism.  In  the  district  farther 
east  there  are  also  some  phenomena  which  seem  to  point  to 
thermal  effects,  e.g.  the  production  of  brown  mica  in  the 
Torridon  Sandstone  near  the  '  Beinn  M6r  thrust-plane.'  But, 
in  proportion  as  the  rocks  affected  give  evidence  by  increasing 
schistosity  of  thorough  mechanical  degradation  and  sliding 
movement,  those  mineralogical  transformations  which  seem  to 
belong  to  pure  dynamic  metamorphism  become  more  general. 
Near  the  great  '  Moine  thrust-plane  '  the  sericitization  of  the 
acid  rocks  and  the  chloritization  of  the  basic  ones  reach  their 
fullest  development  in  connection  with  the  maximum  display 
of  mechanical  deformation.  Detailed  petrographical  observa- 
tions on  this  interesting  district  are  not  yet  forthcoming,  and 
the  same  must  be  said  of  the  region  east  of  the  great  thrust- 
faults,  where  the  complex  of  gneissic  and  other  crystalline 
rocks  known  as  the  *  Moine  schists '  is  supposed  by  some  to 
represent  the  old  gneiss  and  other  rocks  of  the  west  completely 
transformed  by  dynamic  agencies. 

Illustrations  of  dynamic  metamorphism  are  furnished  in 
the  Central  Highlands  and  in  Ireland  by  various  members  of 
the  Dalradian  series  of  Sir  A.  Geikie.  The  so-called  'green 
schists '  are  ascribed  by  that  geologist  partly  to  the  crushing 
of  basic  lavas  and  tuffs.  Some  of  these  rocks  again  have  the 
appearance  of  intrusive  diabases,  in  which  every  stage  of 
crushing  into  chloritic  schists,  etc.,  can  be  traced  (North  Esk, 
Kincardineshire). 

Gradual  transitions  from  massive  diorite  to  hornblende- 
schists  may  be  studied  in  Anglesey,  especially  between  Holland 
Arms  or  Gaerwen  and  Menai  Bridge1.  In  the  processes  by 
which  these  schistose  rocks  have  been  produced  the  felspar 
has  often  been  destroyed,  and  is  represented  in  great  part  by 
epidote,  which  is  often  abundant.  The  granular  sphene,  which 
is  often  seen,  is  probably  derived  in  part  from  ilmenite,  as  well 

1  Blake,  Rep.  Brit.  Ass.  for  1888,  400. 


AMPHIBOLITE-SCHISTS.  325 

as  from  the  original  sphene  of  the  diorite.  The  hornblende  has 
recrystallized  in  imperfect  elongated  crystals  of  green  colour 
with  marked  parallel  orientation.  Locally  the  place  of  this 
mineral  is  taken  by  a  beautiful  pleochroic  glaucophane,  and  a 
rock  near  the  Anglesey  Monument1  is  a  glaucophane-epidote- 
schist,  with  little  trace  of  any  other  mineral,  except  veinlets 
of  clear  secondary  felspar.  The  pleochroism  of  the  glauco- 
phane (bright  blue  to  pale  lilac)  and  the  epidote  (yellowish 
green  to  pale  yellow)  makes  a  slice  of  this  rock  a  very  striking 
object. 

The  name  '  amphibolite '  has  often  been  applied  to  rocks, 
usually  more  or  less  markedly  schistose,  in  which  hornblende 
is  the  dominant  mineral.  Many  of  them  are  doubtless  the 
results  of  dynamic  action  on  diorites  and  sometimes  on  diabases 
and  gabbros.  Two  or  three  types  from  the  Scottish  Highlands 
have  been  figured  by  Mr  Teall,  including  an  epidote-amphibolite 
from  Glen  Lyon,  Perthshire2,  and  a  zoisite-amphibolite  from 
near  Beinn  Hutig,  in  Sutherland3. 

Interesting  phenomena  of  dynamic  metamorphism  have 
been  described  by  Smyth  in  the  gabbros  of  the  Adirondacks 
at  Russell,  St  Lawrence  County,  N.Y.4  The  original  rocks 
consisted  essentially  of  labradorite  and  augite.  From  the 
former  mineral  has  arisen  scapolite  and  sometimes  a  saussurite- 
like  aggregate  ;  from  the  latter  a  scaly  green  hornblende.  In 
a  further  stage  of  alteration  cataclastic  effects  become  marked, 
all  the  constituents  becoming  granulated,  while  the  hornblende 
increases  in  amount.  In  the  final  stage  the  rock  has  taken  on 
a  gneissic  structure,  the  cataclastic  features  are  lost  in  total 
recrystallization,  the  scapolite  has  been  reconverted  to  felspar, 
but  of  a  more  acid  variety  than  the  original  labradorite,  and 
part  of  the  hornblende  seems  to  have  passed  again  into  augite. 

The  'porphyroids '  of  some  authors  are,  for  the  most  part, 
quartz-porphyries  more  or  less  modified  by  dynamic  meta- 
morphism. They  have  received  a  rough  schistosity,  which  is 
accentuated  by  films  of  '  sericitic '  mica,  formed  at  the  expense 

1  Blake,  G.  M.  1888,  125-127 ;  Teall,  pi.  XLVII,  figs.  1,  2. 

2  Teall,  pi.  xxvni,  fig.  2. 

3  Teall,  pi.  XL,  fig.  2.   Cf.  actinolite-schist  with  zoisite,  pi.  xxvin,  fig.  1. 

4  A.J.S.  (1896)  i,  273-281. 


326  PORPHYROIDES,    ETC. 

of  the  felspar.  The  rock  of  Sharpley  Tor  in  Charnwood  Forest 
is  a  good  example.  Similar  features  are  shewn  by  the  Llanberis 
mass  of  quartz-porphyry  at  numerous  points  on  its  south- 
eastern edge,  especially  near  Llanllyfni  (fig.  80,  B).  Some  of  the 
'  porphyroides '  of  the  Meuse  Valley  shew  a  similar  schistose 
structure  with  much  filmy  mica.  The  same  plentiful  produc- 
tion of  sericite  in  connection  with  a  secondary  schistosity  is 
seen  in  the  acid  lavas ;  e.g.  the  old  rhyolites,  compact  and 
spherulitic,  of  the  Lenne,  in  Westphalia1.  A  different  type  is 
illustrated  by  the  rhyolite-gneiss  of  Berlin  in  Wisconsin2. 
Here  the  chief  transformations  to  be  noted  are  the  setting  up 
of  a  microperthitic  structure  in  the  plagioclase  phenocrysts 
and  the  recrystallization  and  orientation  of  the  ground-mass. 

The  phenomena  of  dynamic  metamorphism  in  argillaceous 
sediments  (phyllites,  etc.)  have  received  some  notice  in  a  former 
chapter.  The  other  groups  of  sedimentary  rocks  have  been  less 
studied  from  this  point  of  view.  Some  of  the  phenomena 
observable  in  the  arenaceous  rocks  and  quartzites  of  Suther- 
land3, culminating  in  complete  mylonitization,  we  have  already 
alluded  to.  Interesting  mechanical  effects  are  produced  where 
alternating  gritty  and  slaty  beds  have  been  subjected  to 
crushing.  Some  remarkable  cases  have  been  described  by  Mr 
Lamplugh  in  the  Skiddaw  Slates  of  the  Isle  of  Man  ;  and  Prof. 
Watts  has  shewn  how  the  structures  seen  in  the  field  are 
repeated  on  a  small  scale  in  slices  of  the  rocks4. 

Calcareous  rocks  again  are  susceptible  of  considerable 
transformations,  chiefly  of  the  nature  of  structural  rearrange- 
ment, when  subjected  to  intense  mechanical  forces.  Excellent 
examples  are  afforded  by  the  Ilfracombe  and  other  Devonian 
limestones,  to  which  Dr  Sorby5  drew  attention  many  years 
ago.  These  often  shew,  not  only  a  highly  developed  slaty 
cleavage,  but  also  a  deformation  of  the  individual  fragments 
(such  as  crinoidal  remains,  etc.)  of  which  they  are  largely 
composed,  besides  curious  phenomena  resulting  from  solution 

1  Cf.  Science  Progress  (1894),  ii,  55,  56. 

2  Weidman,  Hull.  Geol.  and  Nat.  Hist.  Sur.  Wis.  No.  in  (1898). 

3  Teall,  pi.  XLVI,  fig.  2. 

4  Q.  J.  G.  S.  (1895)  li,  563-597,  pi.  xx,  xxi. 

6  Phil.  Mag.  (1856)  xi,  26-34;  Presid.  Addr.  1879,  Q.  J.  G.  S.,  xxxv 
(Proc.),  57-59.  See  also  Marr,  G.  M.  1888,  218-221. 


CRUSHED   LIMESTONES. 


327 


having  proceeded  at  the  places  of  greatest  pressure  and  simul- 
taneous crystallization  at  the  places  of  greatest  relief.  The 
cleaved  limestones  near  Ilfracombe  have  a  microscopic  '  eyed ' 
structure,  owing  to  the  preservation  of  uncrushed  lenticles  of 
the  original  rock  (fig.  80,  A).  The  salite-bearing  limestone  of 


B 


FIG.  80.     SCHISTOSE  STRUCTURES  SET  UP  BY  CRUSHING  ;    x  20. 

A.  Devonian    limestone,    Ilfracombe:    with    uncrushed    'eyes'   or 
leuticles  [783]. 

B.  Quartz-porphyry,    Llanllyfni,    Caernarvonshire :    the    schistose 
structure  accentuated  by  films  of  secondary  mica  ('  sericite ')  [87]. 

Tiree1  in  the  Hebrides  also  illustrates  well  the  crushing  of  a 
crystalline  calcareous  rock  and  the  production  of  a  fluxional 
schistose  structure  of  varying  perfection.  This  structure  winds 
past  the  more  resisting  grains  of  salite,  felspar,  etc.  (of  detrital 
origin),  and  in  the  corners  of  the  *  eyes '  so  left  are  uncrushed 
relics  of  the  original  calcite-mosaic. 

1  Bonney,  G.  M.  1889,  485. 


CHAPTER  XXII. 

VARIOUS  CRYSTALLINE  ROCKS. 

IN  this  final  chapter  we  shall  consider  briefly  certain 
groups  of  crystalline  rocks,  some  of  very  wide  distribution, 
the  classificatory  position  of  which  is  in  some  doubt,  owing 
to  divergence  of  opinion  concerning  their  origin.  It  will  be 
evident  on  consideration  that  this  difficulty  arises  in  great 
measure  from  the  grouping  together  under  one  descriptive 
name  and  definition  of  rocks  whose  common  characteristics 
have  originated  in  quite  different  ways.  Until  more  complete 
knowledge  may  lead  to  a  true  genetic  classification,  we  must 
be  content  to  bear  in  mind  that  such  names  as  '  crystalline 
schist,'  *  gneiss,'  and  '  granulite '  do  not  stand  for  natural 
groups,  but  are  of  merely  descriptive  significance ;  and  we 
notice  that  various  examples  of  them  have  already  figured  in 
the  preceding  pages. 

Crystalline  schists.  Under  the  general  title  of  crystal- 
line schists1  (Ger.  krystallinischen  Schiefer)  are  comprised 
rocks  of  distinctly  crystalline  texture  which  possess  a  parallel 
arrangement  of  some  or  all  of  their  elements,  often  with  a 
tendency  to  the  aggregation  of  particular  constituents  into 
streaks  (foliation),  and  which  have  in  consequence  the  property 
of  splitting  with  more  or  less  facility  in  a  definite  direction 
(schistosity). 

1  Many  English  writers  use  the  name  '  schist '  simply  in  this  sense. 
This  practice  is  liable  to  cause  confusion,  since  '  schiste '  is  used  in 
France  (as  formerly  in  this  country)  for  an  ordinary  shale. 


CRYSTALLINE   SCHISTS.  329 

The  structures  due  to  the  parallel  orientation  of  crystals 
are  various,  and  should  be  distinguished.  Mr  Blake l  recognizes 
the  '  quincuncial,'  in  which  the  crystals  which  give  the  structure 
(e.g.  flakes  of  mica)  are  scattered  promiscuously  through  the 
rock,  but  in  parallel  position  ;  the  *  linear,'  in  which  these 
crystals  occur  in  lines,  as  well  as  having  a  general  parallelism  ; 
and  the  '  elemental,'  in  which  the  orientation  is  shewn,  not  by 
some  particular  constituent,  but  by  all  the  elements  of  the 
rock.  Further,  some  degree  of  aggregation  of  the  several 
constituent  minerals  into  streaks  may  give  rise  to  an  inconstant 
banding  or  to  lenticular  structures  (Ger.  'flaser')  on  a  small 
scale.  The  degree  of  schistosity  imparted  by  these  structures 
depends  partly  upon  the  minerals  which  figure  in  them,  being 
most  marked  for  flaky  and  acicular  crystals  (like  mica  and 
actinolite). 

It  must  be  observed,  as  already  pointed  out,  that  the 
meaning  thus  attached  to  the  term  'crystalline  schist'  is  a 
purely  descriptive  one,  founded  upon  structural  features  which, 
as  we  have  already  seen,  may  arise  in  very  diverse  ways.  The 
rocks  included  by  such  a  name  are  not  to  be  regarded  as  a 
natural  group.  A  similar  remark  applies  to  the  special  names, 
mica-schist,  hornblende-schist,  etc.,  used  for  different  kinds  of 
crystalline-schists.  For  another  reason,  too,  such  names  are 
lacking  in  precision,  indicating,  as  they  do,  only  one  of  the 
component  minerals  of  a  complex  rock.  Further  information 
may  be  embodied,  if  necessary,  in  epithets  (e.g.  garnetiferous 
mica-schist)  or  in  compound  names  (e.g.  andalusite-mica- 
schist).  Again,  such  terms  as  diorite-schist  and  limestone- 
schist  are  sometimes  used  to  indicate  that  the  rock  so  named 
has  the  mineralogical  composition  of  a  diorite  or  a  limestone 
with  a  schistose  structure. 

While  much  difference  of  opinion  exists  as  to  the  inter- 
pretation of  particular  areas,  it  is  now  generally  admitted  that 
the  crystalline  schists  as  a  whole  are  metamorphic  rocks  owing 
their  present  distinguishing  characters  in  some  cases  to  thermal, 
in  other  cases  to  dynamic  agency2.  We  have  studied  in  the 

1  Eep.  Brit.  Ass.  for  1888,  379,  figs.  5-7. 

2  For  a  summary  of  views  on  this  question  and  for  much  valuable 
information   the   student   should  consult  the  series  of  papers  on  Les 
Schistes   Cristallins   contributed  by  a  number  of  writers  to  the  Inter- 


330  GNEISSIC   STRUCTURES. 

two  preceding  chapters  numerous  types  of  crystalline  schists 
(as  well  as  non-schistose  rocks)  belonging  to  the  two  divisions 
thus  indicated.  The  facts  there  detailed  enable  us  in  a  con- 
siderable number  of  cases  to  tell  with  some  confidence  from 
what  type  of  original  rock  a  given  crystalline  schist  has  been 
produced,  and  to  ascertain  whether  its  metamorphism  is  the 
result  of  heat  or  of  mechanical  forces.  In  other  cases  one  or 
both  of  these  questions  must  be  left  in  doubt. 

We  may  remark  that,  while  in  rocks  resulting  from  thermal 
metamorphism  foliation  and  schistosity  follow  the  direction  of 
pre-existing  structural  planes  (laminae  of  deposition,  cleavage, 
flow  of  lavas,  etc.),  in  crystalline  schists  due  to  dynamic  agency 
the  new  structures  have  their  direction  determined  by  the 
forces  that  produce  them,  and  tend  to  obliterate,  instead  of 
emphasizing,  any  original  structural  planes  in  the  mass 
affected. 

Gneisses.  The  term  'gneiss'  is  now  used  to  denote,  not 
a  rock  of  some  defined  composition,  but  any  crystalline  rock 
possessing  a  gneissic  structure.  By  this  is  to  be  understood  a 
banded  or  streaky  character  due  to  the  association  or  alterna- 
tion of  different  lithological  types  in  one  rock-mass  or  to  the 
occurrence  of  bands  or  lenticles  specially  rich  in  some  particular 
constituent  of  the  rock.  The  structure  is  often  found  on  a 
relatively  coarse  scale  in  rocks  of  granitoid  texture,  so  that  it 
is  to  be  observed  rather  in  the  field  or  in  large  specimens  than 
in  microscopical  preparations.  It  may,  however,  be  associated 
with  foliation  on  a  smaller  scale  or  with  a  partial  parallel  dis- 
position of  the  elements  of  the  rock.  Gneisses,  in  this  sense, 
may  have  the  chemical  and  mineralogical  composition  of  acid 
or  intermediate  or  basic  rocks,  or  may  belong  to  types  without 
parallel  among  the  known  products  of  igneous  magmas. 

It  is  generally  recognized  that  gneisses  as  thus  defined  have 
originated  in  more  than  one  way,  but  much  difference  of  opinion 
exists  as  to  the  interpretation  of  the  facts  in  particular  districts. 
We  shall  note  here  the  three  cases  which  are  probably  of  the 
most  general  importance. 

national  Congress  of  Geologists  at  London,  1888;  pp.  65-102  of  the 
Compte  Eendu  (1891).  The  French  and  German  contributions  are 
translated  in  Nature,  Sept.  20,  27,  Oct.  4  (1888). 


GNEISSJC    STRUCTURES.  331 

(i)  We  have  already  seen  that  gneisses  may  originate  by 
the  thermal  metamorphism  of  some  sedimentary  (and  volcanic) 
rocks.  The  New  Galloway  rocks  and  the  staurolite-,  cyanite-, 
and  sillimanite- bearing  gneisses  of  the  South-eastern  Highlands 
are  examples.  The  abundance  of  aluminous  silicates  is  charact- 
eristic, and  so  also  is  quartz  as  an  essential  constituent  in 
rocks  with  only  a  low  percentage  of  silica.  Under  this  head 
are  probably  to  be  included  such  rocks  as  the  biotite-gneiss  of 
the  Black  Forest  and  the  rock  known  as  '  kinzigite,'  consisting 
essentially  of  garnet,  biotite,  and  plagioclase,  besides  the  horn- 
blende-gneisses of  the  Odenwald,  the  Wahsatch,  etc.  All  these 
have  the  chemical  composition  of  sedimentary  rocks ;  Rosen- 
busch  styles  them  *  paragneisses,'  in  contra- distinction  to 
'  orthogneisses,'  which  have  the  composition  of,  and  are  believed 
to  represent,  igneous  rocks.  In  these  latter  there  remain,  as 
we  shall  see,  two  possible  explanations  of  the  gneissic  structure. 

(ii)  It  appears  that  gneissic  banding  may  be  set  up,  more 
particularly  in  plutonic  rocks,  by  dynamic  agency,  i.e.  by  the 
mechanical  deformation  of  a  rock-mass  originally  heterogeneous 
or  of  a  complex  in  which  one  rock  was  traversed  or  veined  by  a 
different  one.  In  such  a  case  we  should  expect  to  find  further 
some  degree  of  foliation  and  schistosity  and  usually  lenticular 
structures,  quasi-porphyritic  '  eyes,'  or  other  characteristic  feat- 
ures. Numerous  examples  have  been  cited  by  Reusch  from 
the  western  coast  of  Norway  and  by  other  observers  elsewhere. 
Mr  Teall1  has  applied  the  hypothesis  of  mechanical  deformation 
to  gabbros,  granites,  and  diorites  with  gneissic  and  schistose 
structures  in  the  Lizard  district.  Gen.  McMahon2,  on  the 
other  hand,  considers  that  these  structures  were  impressed 
on  the  rocks  while  still  only  partially  consolidated.  He 
compares  the  Lizard  rocks  with  the  gneissic  granites  about 
Dalhousie,  etc.,  in  the  Himalaya  region3,  which  he  believes  to 
have  been  intruded  in  a  partially  consolidated  state  and  to 
have  assumed  at  that  time  their  gneissic  and  foliated  structures. 
Mr  Middlemiss4,  however,  ascribes  these  structures  in  the 

1  G.  M.  188&,  481-489  ;  1887,  484-493. 

2  G.  M.  1887,  74-77. 

3  G.  M.  1887,  212-220 ;  1888,  61-65. 

4  Mem.  Geol.  Sur.  Ind.  (1896)  xxvi,  65,  etc. 


332  PRIMARY    BANDING    IN    GNEISSES. 

Himalayan  gneisses  to  dynamic  metamorphism  operating  on 
the  solid  rocks. 

(iii)  There  is  no  doubt  that  gneissic  banding  may  be  an 
original  character  in  plutonic  rocks,  dating  from  the  time 
when  the  rock  in  question  was  still  fluid  or  partly  fluid,  and 
due  to  the  different  portions  of  a  heterogeneous  magma  being 
drawn  out  in  a  flowing  movement.  A  remarkable  example  is 
described  by  Sir  A.  Geikie  and  Mr  Teall1  in  certain  Tertiary 
gabbros  in  Skye.  These  rocks  shew  a  striking  alternation  of 
light  and  dark  bands  due  to  differences  in  the  relative  propor- 
tions of  the  constituent  minerals  of  the  gabbro  (labradorite, 
augite,  olivine,  and  titaniferous  magnetite).  Some  narrow 
bands  are  composed  entirely  of  pyroxene  and  iron-ore.  The 
authors  compare  these  rocks  with  the  '  Norian  '  gabbros  and 
anorthosites  of  North  America  and,  as  regards  structures,  with 
the  Lewisian  gneisses  of  the  North-west  Highlands. 

These  latter,  apart  from  the  innumerable  dykes  by  which 
they  are  traversed,  present  much  variation  in  character.  In 
the  north,  between  Cape  Wrath  and  Loch  Laxford,  hornblendic 
and  micaceous  gneisses  predominate.  From  Scourie  to  beyond 
Lochinver  and  Loch  Assynt  the  prevalent  type  is  a  pyroxenic 
gneiss2,  consisting  essentially  of  augite  or  hypersthene  (Kyle- 
sku),  felspars,  and  quartz.  There  are  also  acid  types,  consisting 
mainly  of  felspars  and  quartz  ;  while,  on  the  other  hand,  the 
dominant  rock  encloses  portions  very  rich  in  green  hornblende. 
Hornblendic  and  micaceous  gneisses  predominate  again  about 
Gairloch  and  Loch  Torridon,  and  a  coarse  hornblendic  gneiss 
occurs  in  Lewis  (Stornoway)  besides  other  types.  Many  of  these 
rocks  shew  in  varying  degree  the  effects  of  dynamic  metamorph- 
ism, but  the  authors  named  consider  that  much  of  the  banding 
(as  distinguished  from  foliation)  may  be  ascribed  to  original 
conditions  attending  the  intrusion  of  igneous  magmas. 

In  the  South  eastern  Highlands  (Forfarshire  and  Kincard- 
ineshire)  Mr  Barrow3  has  described  certain  micaceous  gneisses 
which  are  clearly  igneous  intrusions  separable  from  the  meta- 
morphic  gneisses,  alluded  to  above,  with  which  they  are 

1  Q.  J.  G.  S,  (1894)  1,  645-659. 

2  Teall,  pi.  XL,  fig.  1. 

3  G.  M.  1892,  64,  65 ;  Q.  J.  G.  S.  (1893)  xlix,  330-335. 


EXAMPLES   OF  GNEISSIC   ROCKS.  333 

associated.  In  one  phase  the  rocks  consist  essentially  of  quartz, 
peculiar  rounded  crystals  of  oligoclase,  muscovite,  and  biotite. 
Another  phase  shews  abundant  microcline,  with  a  corresponding 
diminution  of  oligoclase,  while  at  the  same  time  the  white 
mica  predominates  increasingly  over  the  brown,  and  builds 
larger  crystals.  The  author  makes  it  clear  that  the  remarkable 
features  of  these  igneous  gneisses  are  due  in  the  main  to  crust- 
movements  at  the  epoch  of  intrusion. 

Prof.  F.  D.  Adams1  has  described  a  number  of  Canadian 
gneisses,  some  of  igneous  origin  and  affected  by  dynamic 
metamorphism,  others  sediments  altered  by  thermal  meta- 
morphism,  and  he  has  pointed  out  criteria  for  discriminating 
the  two. 

Although  we  have  distinguished  primary  and  secondary 
banding  and  foliation  in  plutonic  rocks,  the  alternatives 
numbered  (ii)  and  (iii)  above  are  by  no  means  mutually 
exclusive  in  application  to  any  given  rock,  and  a  gneiss  of 
primary  igneous  origin  may  present  phenomena  due  to 
subsequent  dynamic  metamorphism. 

In  the  South  Indian  '  charnockites '  (pyroxene- gneisses  or 
pyroxene-granulites  of  some  authors),  already  referred  to 
under  the  head  of  hypersthene  granite,  Mr  Holland  has  shewn 
that  the  frequent  banding  and  foliation  are  primary,  but 
dynamic  effects  are  also  indicated,  notably  in  the  production 
of  garnet.  Lacroix2  noted  that  in  these  rocks  the  felspars 
are  often  crowded  with  little  round  or  elongated  inclusions  of 
quartz  ('quartz  de  corrosion'  of  French  writers)  without  the 
regularity  of  a  graphic  intergrowth.  This  is  ascribed  to 
secondary  corrosion. 

The  associated  basic  rocks  present  in  some  cases  more 
difficult  problems.  Here  are  found  curious  micrographic 
intergrowths  between  the  ferro-magnesian  minerals  (pyroxene, 
hornblende,  garnet)  on  the  one  hand  and  felspar  and  quartz 
on  the  other.  Lacroix  finds  scapolite  a  characteristic  consti- 
tuent of  the  *  pyroxene-gneisses  '  here  and  in  other  districts 3. 

1  Ann.  Rep.  Geol.  Sur.  Can.  (1895)  viii,  31-81  J,  pi.  iv,  v. 

2  Rec.  Geol.  Sur.  Ind.  (1891)  xxiv,  157-190. 

3  Cf.  Judd  and  Brown,  Proc.  Roy.  Soc.  (1895)  Ivii,  391 ;  Phil.  Trans, 
(1896)  clxxxvii,  A,  193-204,  pi.  vi  (Burma). 


334 


GRANULITES. 


Granulites.  (Fr.  leptyriites1.)  The  granulites  are  fine- 
textured  crystalline  rocks  consisting  of  quartz,  felspars,  and 
various  other  minerals,  among  which  garnet  is  highly  charact- 
eristic. They  shew  a  remarkable  uniformity  of  grain  among 
the  several  constituents.  There  is  often  a  more  or  less 
evident  parallel  orientation  of  the  elements,  but  no  schist- 
psity.  Such  an  even-grained  mosiac  we  have  already  noticed 
in  some  of  the  products  of  extreme  thermal  metamorphism 
and  again  in  the  rocks  resulting  from  the  *  granulitization '  of 
crystalline  masses  in  connection  with  crushing,  while  some- 
what similar  features  are  found  in  rocks  formed  directly  from 
igneous  fusion.  Indeed  any  petrographical  definition  of 
granulite  will  be  found  to  cover  rocks  having  quite  different 
origins. 

It  will  be  sufficient  to  notice  briefly  some  of  the  characters 
of  the  more  or  less  indefinite  group  of  rocks  known  as  granul- 
ites in  Saxony  and  other  parts  of  Europe,  where  they  attain 
a  very  considerable  development.  These  rocks  have  provoked 
much  difference  of  opinion,  but  it  is  now  generally  believed 
that  many  of  them  are  of  igneous  origin,  while  they  often 
bear  evidence  of  the  operation  of  mechanical  forces  either 
during  or  after  their  formation.  The  varieties  of  most 
common  occurrence  are  acid  rocks,  but  there  is  also  a 
division  of  basic  composition  (pyroxene-gran ulite,  or  Trapp- 
granulit  of  some  German  writers). 

The  former  contain,  in  addition  to  quartz,  various  alkali- 
felspars — orthoclase,  microcline,  and  plagioclase,  with  some- 
times microperthitic  intergrowths.  Dark  mica  is  commoner 
than  white  as  an  original  constituent,  but  red  garnet  is  more 
prominent  than  either  in  the  usual  types  of  granulites.  All 
these  minerals  occur  in  little  irregular  grains,  usually  clear 
except  for  inclusions  of  earlier  formed  constituents.  Cyanite, 
in  rude  crystals,  sillimanite  prisms  or  aggregates  (fibrolite), 
green  hornblende,  tourmaline,  and  other  minerals  may  occur, 
and  are  taken  as  marking  different  types  (cyanite-granulite, 
tourmaline-granulite,  etc.}.  Many  of  these  rocks  also  contain 


1  The  'granulite'  of  French  writers  signifies  a  granite  with  white 
and  dark  micas. 


GARNET-GRANULITES. 


335 


garnet ;  and  garnet-granulite,  in  which  that  mineral  is  the 
characteristic  one,  is  the  most  familiar  type  (Chemnitz  district 
in  Saxony,  Wartha  in  Bohemia,  Nanniest  in  Moravia,  '  leptyn- 

ites  '  of  the  Vosges,  etc.  :  see  fig.  81). 


FIG.  81.     GARNET-GRANULITE,  ROHRSDORF,  NEAR  CHEMNITZ, 
SAXONY  ;    x  20. 

Shewing  grains  of  garnet  (g)  and  imperfect  prisms  of  cyanite  (cy)  set 
in  a  granular  aggregate  of  felspar  and  quartz.  The  latter  shews  a 
parallel  arrangement  of  its  larger  elements,  and  there  are  rows  of  fluid- 
pores  traversing  the  rock  at  right  angles  to  the  parallel-structure  [835]. 

The  pyroxene-granulites  are  rich  in  irregular,  often  rounded, 
grains  of  pyroxene  in  addition  to  quartz,  plagioclase  (usually 
not  orthoclase),  often  garnet,  biotite,  and  magnetite.  The 
pyroxenes  include  apparently  both  hypersthene  and  a  pleo- 
chroic  (pink  to  pale  green)  augite  closely  resembling  it 
(Mohsdorf,  Hartmannsdorf,  etc.,  in  Saxony). 

A  frequent  peculiarity  in  all  the  granulites  is  the  occur- 
rence of  what  have  been  styled  centric  structures1,  of  which 

1  Cohen  (3),  pi.  xxxvm,  figs.  1,  2. 


336  PYROXENE-GRANULITES. 

the  most  usual  take  the  form  of  aggregates  of  various  con- 
stituents about  the  grains  of  garnet,  or  radial  groupings  of 
such  minerals  as  pyroxene  or  hornblende,  with  or  without 
a  garnet  in  the  centre  (cf.  fig.  82). 


FIG.  82.     PYROXENE-GRAKULITE,  CHEMNITZBACH,  NEAR  MOHSDORF, 

SAXONY  ;     x  20. 
» 

Much  of  the  pyroxene  is  hypersthene :  the  clear  portion  of  the  slice 
is  a  mosaic  of  plagioclase  felspar  and  quartz.  The  rock  shews  a  rude 
'  centric '  structure  in  the  arrangement  of  the  pyroxene-grains  [494  a]. 

In  granulites  having  an  evident  parallel-structure  there 
are  often  lines  of  fluid-pores  arranged  transversely  to  that 
structure  and  passing  through  the  quartz  and  felspar  alike 
(fig.  81).  This  may  be  noticed  as  a  well-known  strain- effect.. 
Strain-shadows  in  the  crystals  are  not  often  observed  in 
granulites.  In  some  of  the  rocks  (Ger.  Augengranulit)  there 
are  '  eyes '  consisting  of  larger  lenticular  individuals  of  felspar 
or  quartz-felspar  aggregates. 

Of  rocks  which  may  be  petrographically  described  as 
granulites  numerous  examples  are  found  in  the  Scottish 
Highlands.  Garnet-granulites  are  represented,  one  type  con- 
sisting of  quartz,  felspars,  garnet,  and  biotite,  with  a  little 
muscovite,  sphene,  and  magnetite  (e.g.  Beinn  Wyvis).  Actin- 


MINERALS   OF    ECLOGITES.  337 

olite-granulites  occur,  shewing  long  imperfect  prisms  of  green 
actinolite  in  a  clear  even-textured  mosaic  of  untwinned 
felspar  and  quartz,  with  a  little  magnetite  and  small  flakes  of 
biotite  (Strathan  near  Lochinver). 

Pyroxene-granulites  are  also  found  :  one  type  consists  of 
diallagic  augite,  sometimes  hypersthene,  and  abundant  clear 
plagioclase,  with  some  biotite  and  magnetite.  Garnet  is  only 
sparingly  present,  and  there  is  very  little  quartz  (Badenaban 
near  Lochinver).  Professor  Cole1  has  figured  a  pyroxene- 
granulite  with  hypersthene  and  garnet  from  near  Huntley, 
Aberdeenshire. 

Eclogites.  Among  rocks  of  somewhat  doubtful  affinities 
must  be  mentioned  the  small  group  of  the  eclogites.  The 
typical  eclogite  of  Haiiy  consists  essentially  of  an  aluminous 
augite  (omphacite)  and  red  garnet,  with  sometimes  quartz, 
hornblende,  actinolite  (smaragdite),  cyanite,  or  other  acces- 
sories. From  their  mode  of  occurrence,  the  rocks  are  commonly 
regarded  as  of  true  igneous  origin. 

The  dodecahedral  or  rounded  crystals  of  garnet  are  quite 
pale  in  thin  slices.  They  contain  various  inclusions,  such  as 
quartz  granules  (collected  in  the  centre  of  the  crystal),  needles 
of  cyanite2  or  rutile  (ranged  in  rows  parallel  to  the  faces 
of  the  dodecahedron),  little  prisms  of  zircon,  etc. 

The  green  omphacite  is  nearly  colourless  in  slices.  It 
builds  columnar  crystals,  which,  when  moulded  by  quartz, 
may  have  good  faces,  but  usually  build  an  irregular  aggregate 
or  shew  a  parallel  arrangement.  Besides  the  prismatic 
cleavage  there  may  be  one  parallel  to  the  orthopinacoid,  or  a 
slight  diallagic  structure.  The  extinction-angle  rises  to  40° 
or  more.  Inclusions  of  rutile,  etc.,  are  found,  and  sometimes 
a  parallel  intergrowth  of  bright  green  smaragdite.  Some 
eclogites  contain  a  pale  yellowish  green  bronzite  :  this  forms 
idiomorphic  crystals. 

Clear  quartz  is  usually  present ;  biotite  flakes  sometimes 
cling  about  the  garnet  crystals  ;  while  cyanite,  zoisite,  glauco- 

1  Aids  in  Pract.  Geol.  (1891)  p.  210. 

2  G.  M.  1891,  170,  171  (Port  Tana,  Norway). 

H.  P.  22 


338 


STRUCTURE   OF    ECLOGITES. 


phane,  zircon,  rutile,   etc,,  may  be  seen  in  some  examples. 
Iron-ores  are  not  abundant. 

The  omphacite  makes  up  the  bulk  of  the  rock,  forming  a 
crystalline  aggregate  in  which  the  garnet  is  embedded,  while 
quartz  is  always  of  interstitial  occurrence.  A  clear  ring  or 
shell  of  the  last  mineral  is  often  interposed  between  each 
garnet  and  the  surrounding  omphacite.  Again,  the  garnet  is 
sometimes  broadly  bordered  by  a  '  cely phi te' -growth  with 


f 


FlG.    83.       HORNBLENDE-ECLOGITE    (GABNET-AMPHIBOLITE),    LOCH 

LAXFORD,  SUTHERLAND  ;     x  15. 

Consisting  of  red  garnet  (g)  and  green  hornblende  (h),  with  only  a 
little  clear  quartz,  turbid  felspar  (/),  and  opaque  iron-ore.  The  arrows 
shew  the  directions  of  the  stresses  that  have  operated  in  the  rock,  and 
the  brittle  garnets  are  traversed  by  a  strongly  marked  system  of  cracks 
perpendicular  to  the  direction  of  tension  [1254]. 

radial  or  plumose  arrangement  and  of  varying  constitution. 
In  a  Bohemian  example  (Chlumicek)  it  consists  of  radiating 


ECLOGITES   AND   HORNBLENDE-ECLOGITES.  339 

bundles  of  enstatite  prisms  :  in  other  cases  actinolite,  biotite, 
and  other  minerals  take  part  in  the  celyphite-border. 

The  best  known  eclogites  are  from  Bavaria1  (Eppenreuth, 
with  cyanite,  etc.,  Silberbach),  the  Saxon  Granulite  Mount- 
ains (Waldheim,  with  sphene),  Silesia  (Frankenstein,  with 
zoisite  supposed  to  be  derived  from  the  garnet),  the  Pennine 
Alps  (Val  d'Aoste2,  with  glaucophane),  Carinthia  (Saualp3, 
with  zoisite),  the  island  of  Syra  (with  glaucophane),  etc. 
Good  examples  of  eclogite  occur  in  the  district  of  Loch  Duich 
and  Glenelg,  on  the  west  coast  of  Scotland  ;  and  one  of  these, 
from  Totaig,  has  been  described  by  Mr  Teall4.  This  contains 
green  hornblende,  partly  surrounding  the  garnet,  and,  instead 
of  quartz,  a  plagioclase  felspar  occurs  in  small  quantity  inter- 
stitially  or  in  micrographic  intergrowth  with  the  omphacite. 
A  rock  comparable  in  many  respects  with  eclogite  is  recorded 
from  Mountain  Lodge,  near  Pettigo,  Donegal5. 

Closely  allied  to  the  typical  eclogites  are  the  rocks  styled 
garnet-amphibolite ;  in  which  hornblende  more  or  less  com- 
pletely takes  the  place  of  omphacite.  Such  rocks  are  found  in 
Norway,  Silesia,  and  other  areas.  Prof.  Bonney6  has  described 
one  from  Beinn  Fyn,  near  Loch  Maree,  under  the  name  horn- 
blende-eclogite.  It  consists  mainly  of  garnet  and  green  horn- 
blende with  some  quartz,  plagioclase,  etc.  A  handsome  rock 
having  the  same  general  characters  occurs  near  Loch  Laxford, 
in  Sutherland7  (fig.  83).  It  will  be  seen  that  the  only 
members  of  the  eclogite  group  yet  recognized  in  Britain  are 
all  associated  with  the  old  gneisses  of  the  Highlands.  A  rock 
consisting  of  garnet  and  greenish  brown  hornblende,  with 
grains  of  rutile,  has  been  noted  from  Santa  Catalina  Is., 
Cal.8 

i.Newland,  Trans.  N.Y.  Acad.  Sci.  (1897)  xvi,  24-29;  Cohen  (3), 
pi.  xvn,  fig.  1. 

2  Bonney,  M.  M.  (1886)  vii,  1-3,  pi.  i,  and  Phil.  Mag.  (1892)  xxxiii, 
244. 

3  Cohen  (3),  pi.  xxi,  fig.  1. 

4  M.  M.  (1891)  ix,  217,  218. 

5  Cole,  Tr.  Roy.  Ir.  Acad.  (1900)  xxxi,  457,  458,  pi.  xxvi,  fig.  6. 
«  Q.  J.  G.  S.  (1880)  xxxvi,  105,  106. 

7  G.  M.  1891,  171,  172. 

8  W.  S.  T.  Smith,  Pr.  Cal.  Acad.  Sci.  (1897)  i,  62-64. 


340  GLAUCOPHANE-ECLOGITE. 

In  a  French  hornblende- eclogite,  a  local  modification  of  a 
diorite,  the  hornblende  is  light  brown1.  In  other  examples 
the  amphibole  is  a  glaucophane  with  vivid  blue  and  violet 
pleochroism  (Val  d'Aoste  in  Pennine  Alps,  He  de  Groix  in 
Brittany)2. 

1  Fouque  and  L£vy,  pi.  vi. 

2  Bonney,  M.  M.  (1886)  vii,  1-7,  150-154,  pi.  i. 


INDEX. 


[Some  rock-names  are  given  here  ivhich  are  not  admitted  into  the  text. 
The  list  will  thus  serve  to  some  extent  as  a  glossary.] 


Absarokite  (Iddings),  209 
Absorption  colours,  3 
Abyssal  rocks,  23;   clay,  242 
Acid  excretions,  26 
Acid  intrusives,  103 
Acrnite-trachyte  (Wolff  and  Tarr), 

122 

Adamellite  (Brogger),  62 
Adinole  (Haussmann),  304 
Ailsite    (Blackwood    and   Heddle), 

X16 

Akerite  (Brogger),  49 
Albite-porphyry,   123 
Allotbigenous  (Kalkowsky),   223 
Allotriomorphic  (Rosenbusch),  23 
Alnoite  (Rosenbusch),  211 
Amphibolite  (Brongniart),  325 
Amygdaloidal,  157;  184 
Analcime-basalt  (Lindgren),  150 
Analcime-diabase,  140 
Andesite  (v.  Buch),  181 
Anhedral  (Pirsson),  23 
Anhydrite,  282 

Anorthosite  (Sterry  Hunt),  84 
Apachite  (Osann),'  180 
Aplite  (Retz),  40 
Aporhyolite  (Bascom),  159 
Arenaceous  rocks,  223 
Argillaceous  rocks,  237 
Arkite  (Washington),  56 
Arkose  (Brongniart),  223 
Ash  (volcanic),  272 
Augenstructur,  318 


Ausweichungsclivage  (Heim),  241 
Authigenous  (Kalkowsky),  223 
Automorphic  (Rohrbach),  23 
Axiolite  (Zirkel),  161 

Banakite  (Iddings),  209 

Barkevicite,  46 

Basalt,  194 

Basanite,  210 

Basic  secretions,  26,  43 

Bastite,  73 

Bauxite,  243 

Belonite  (Airport),  114 

Birefringence,  15;   table,  17 

Bogenstructur  (Miigge),  274 

Borolanite  (Teall),  55 

Bostonite  (Rosenbusch),  123 

Brachiopods,  252 

Brecciation,  318 

Brucite,  294,  308 

Calcareous  algae,  249,  256 
Calcareous  rocks,  248 
Camptonite  (Rosenbusch),  141,  148 
Carclazite  (Collins),  243 
Carmeloite  (Lawson),  196 
Cataclastic  (Kjerulf),  318 
Celyphite  (Schrauf),  77,  91 
Centric  structure,  108,  335 
Cephalopods,  254 
Ceratophyre  (v.  Giimbel),  119,  125 
Chalk,  270 
Chalybite,  262,  284 

22—3 


342 


INDEX. 


Charnockite  (Holland),  39,  333 
Chert,  285 
Chiastolite,  291 
China-clay,  243 
Chondri,  91 

Ciminite  (Washington),  176 
Cipollino,  306 

Classification  of  igneous  rocks,  20 
Clastic,  222 
Clay,  237,  244 
Clay-slate-needles,  239 
Cleavage-angle,  3 
Cleavage-flakes,  9,  11 
Cleavage-traces,  2 
Cleavage  of  slates,  240 
Close-joints-cleavage  (Sorby),  241 
Coccolith  (Huxley),  259 
Colour  of  minerals,  3 
Comendite  (Bertolio),  169 
Conchite  (Kelly),  248 
Corals,  251 

Cordierite,  292,  297,  300 
Corneenne,  302 
Corona-structure,  77 
Corsite  (Collomb),  62 
Cortlandtite  (G.  H.  Williams),  92 
Covite  (Washington),  51 
Crustacea,  252 
Cryptocrystalline,  106,  158 
Cryptographic  (Harker),  109 
Cryptoperthite  (Brogger),  45 
Crystalline  schist,  328 
Crystallite  (Hall),  113,  157 
Crystallographic  systems,  10 
Cumulite  (Vogelsang),  157 
Cyanite,  292 

Dacite  (Stache),  181,  185 
Decreasing  basicity,  24 
Dedolomitization,  307 
Desmoisite  (Zincken),  302 
Devitrification,  106,  158 
Diabase  (Brongniart),  130 
Diallage-rock,  85 
Diatoms,  285 
Dichroism,  19 
Diorite  (Haiiy),  57 
Diorite-porphyrite,  119,  129 
Ditroite  (Zirkel),  54 
Dolerite  (Haiiy),  194,  200 


Dolomite,  248  ;  dolomitization,  260 

Domite  (v.  Buch),  173 

Drusy  structure,  25 

Dunite  (Hochstetter),  86,  96 

Dyke-rocks,  102 

Dynamic  metamorphism,  287,  315 

Echinoderms,   251 
Eclogite  (Haiiy),  337 
Effusive  (Rosenbusch),  152 
Elaeolite-syenite  (Blum),  45 
Ellipsoid  of  elasticity,   7 
Elvan,  110 

Epidiorite  (v.  Giimbel),  131 
Epidote,  293 

Ergussgesteine  (Rosenbusch),  102 
Essexite  (Sears),  69 
Eucrite  (Rose),  70 
Eugranitic,  25 
Eulysite  (Erdmann),  86,  97 
Even-grained,  27 

Extinction-angles,  9;  ofplagioclase, 
table,  12 

False  cleavage,  241,  246 

Faserkiesel,  292 

Felsite  (Gerhard),  104 

Felsophyre  (Rosenbusch),  106 

Felspar-rock,  84 

Fire-clay,  244 

Flaserstructur  (Naumann),  329 

Fluxion- structure,  151,  155,  332 

Foliation  (Darwin),  328,  330 

Foraminifera,  250 

Forellenstein  (v.  Rath),  85 

Forsterite,  294 

Fourchite  (J.  F.  Williams),  150 

Foyaite  (Blum),  53 

Gabbro  (v.  Buch),  70,  78 
Ganggesteine  (Rosenbusch),  102 
Garnet-amphibolite,  339 
Gasteropods,  254 
Geyserite,  286 
Girvanella,  256 
Glaucon'ite,  232,  238,  270 
Glaucophane-eclogite,  339 
Glaucophane-schist,  304,  325 
Gliding-planes,  316 
Globigerina-ooze,  259 


INDEX. 


343 


Globulite  (Vogelsang),  157 
Glomeroporphyritic  (Judd),  200 
Gneiss,  25,  295,  330 
Granite,  28 
Granite-porphyry,  106 
Granitite  (Eose),  36 
Granitoid  structure,  24 
Granodiorite  (Becker,  Turner,  and 

Lindgren),  64 
Granolite  (Pirsson),  25 
Granophyre     (Vogelsang,     Eosen- 

buscb),  107 

Granophyre-groups  (Iddings),  162 
Granulite  (Weiss),  34,  334 
Granulitic  structure,  25,  134,  203 
Graphic  structure,  25 
Greensand,  231 
Greenstone,  131 
Greisen,  42 
Greywacke,  223 
Grit,  223 

Grorudite  (Brogger),  117 
Grossularite,  293,  307 
Ground-mass,  152 
Gypsum,  282 

Halleflinta,  276 
Haloes  (pleochroic),  31 
Haplite  =  aplite,  40 
Harzburgite  (Eosenbusch),  86,  95 
Haiiynophyre  (Eammelsberg),  215 
Heronite  (Coleman),  126 
Herring-bone  structure,  72 
Holocrystalline,  23,  199 
Hornblende-eclogite,  339 
Hornblende-schist,  322 
Hornfels,  302 
Hornstone,  302 
Hudsonite  (Cohen),  92 
Hyalomicte  (Brongniart),  42 
Hyalopilitic  (Eosenbusch),  184 
Hypabyssal  (Brogger),  102 
Hyperite  (Tornebohm),  82 
Hypersthenite  (Eose),  70 
Hypidiomorphic  (Eosenbusch),  23 
Hypocrystalline,  198 

Iddingsite  (Lawson),  196 
Idiomorphic  (Eosenbusch),  23 
Idocrase,  293 


Ijolite  (Eamsay  and  Berghell),  54 
Inclusions  in  crystals,  5 
Inset  (Blake),  152 
Interference-tints,  15 ;    table,  17 
Intersertal  (Eosenbusch),  192 
Intratelluric  (Eosenbusch),  152 
Iron-ore-rocks,  36 
Ironstone,  262,  284 
Isotropic,  8 

Kalksilicathornfels,  306 

Kaolin,  243 

Kentallenite  (Hill  and  Kynaston), 

51 

Keratophyr  (v.  Giimbel),  119 
Kersantite  (Delesse),  141,  147 
Kieselguhr,  285 
Kieselschiefer,  285 
Kinzigite  (Fischer),  331 
Knitted  structure,  74 
Knotenschiefer,  298 

Labradorite    (lava)     (Fouque    and 

Levy),  182 
Lamellibranchs,  253 
Lamprophyre  (v.  Giimbel),   141 
Lapilli,  271 

Laterite  (Buchanan),  243 
Lattice-structure,  74 
Laurdalite  (Brogger),  53 
Laurvikite  (Brogger),  51 
Leptynite  (Haiiy),  334 
Leucite-basalt  (Zirkel),  210,  218 
Leucite-basanite,  210,  214 
Leucite-syenite,  45 
Leucite-tephrite,  210,  213 
Leucitite,  210,  216 
Leucitophyre  (Coquand),  170,  180 
Leucoxene,  133 
Lherzolite  (Lelievre),  86,  95 
Limburgite  (Eosenbusch),  194,  208 
Lime-silicate-rock,  306 
Limestone,  248 
Lindoite  (Brogger),  122 
Liparite  (Eoth),  154 
Litchfieldite  (Bayley),  54 
Lithophyse  (v.  Eichthofen),  160 
Longulite  (Vogelsang),  158 
Luxulyanite  (Pisani),  41 


344 


INDEX. 


Madupite  (Cross),  218 
Magma-basalt  (Boricky),  208 
Mandelstein,  184 
Margarite  (Vogelsang),  157 
Melaphyre  (Brongniart),  181 
Melilite-basalt  (Stelzner),  210,  220 
Melilite-monchiquite  (Flett),  221 
Mesh-structure,  74 
Metamorphism,  287 
Metasomatism,  260,  287 
Meteorites,  88 
Miarolitic,  25 
Miascite  (Rose),  54 
Mica-lamprophyre,  141,  146 
Mica-schist,  228,  298,  329 
Mica-trap,  141 
Microcline,  29,  317 
Microfelsitic,  106,  158 
Microgranite  (Eosenbusch),  106 
Microgranulitic,  203 
Micrographic,  25,  107 
Microlite  (Vogelsang),   114 
Micropegmatite,  107 
Micropegmatite-phenocryst,  162 
Microperthite,  45 
Micropcecilitic   (Of.    H.    Williams), 

163 

Microspherulitic,  109,  161 
Minette  (Voltz),  141,  147 
Missourite  (Pirsson),  84 
Monchiquite    (Hunter  and  Eoseu- 

busch),  142,  149 
Monzonite  (de  Lapparent),  50 
Mortelstructur  (Tornebohm),  318 
Mylonite  (Lap worth),  318 

Napoleonite,  62 
Neovolcanic,  152 
Nepheline-basalt,  210,  220 
Nepheline-basanite,  210,  216 
Nepheline-dolerite,  218 
Nepheline-syenite,  45 
Nepheline-tephrite,  210,  215 
Nephelinite,  210,  218 
Nevadite  (v.  Bichthofen),  163,  186 
Newton's  scale  of  colours,  16 
Nodular  rhyolites,  161,  167 
Nomenclature  of  rocks,  20 
Nordmarkite  (Brogger),  49 
Norite  (Esmark),  70,  82 


Oblique  extinction,  9 

Obsidian,  156,  164 

Olivine-nodules,  200 

Olivine-rock,  96 

Omphacite,  293,  337 

Oolitic  structure,  255 

Ooze,  259 

Opal,  173,  240 

Ophicalcite,  308 

Ophitic,  134,  203 

Optic  axes,  8 

Orbicular,  84,  62 

Order  of  crystallization,  24 

Orendite  (Cross),  217 

Orthoclase-porphyry,  118,  122 

Orthogneiss  (Eosenbusch),  331 

Orthophyre  (Coquand),  122 

Orthophyric  structure,  121,  174 

Ottrelite,  291 

Ouachitite  (Kemp),  150 

Paisanite  (Osaun),  116 

Palaeo  volcanic,  152 

Palagonite  (v.  Walter shausen),  279 

Panidiomorphic  (Eosenbusch),  144 

Pantellarite  (Forstner),  154,  169 

Paragneiss  (Eosenbusch),  331 

Peg-structure,  221 

Pegmatite  (Haiiy),  40 

Pegmatitic  structure,  26 

Pencatite,  308 

Peridotite  (Eosenbusch),  87,  94 

Perlite  (Bendant),  165 

Perlitic  structure,  156 

Petuntzite  (Collins),  243 

Phenocryst  (Iddings),  152 

Phonolite  (Klaproth),   170,  177 

Phosphatization,  264 

Phthanite,  285 

Phyllade  (Brochant  and  d'Aubuis- 

son),  237 

Phyllite  (Naumann),  237,  245 
Picrite  (Tschermak),  86,  91 
Pilotaxitic  (Eosenbusch),  121,  184 
Pinite,  32,  105 
Pisolite,  255 

Pitchstone,  106,  113,  158,  185 
Plagioclase  felspars  distinguished, 

11 
Plauenite  (Brogger),  47 


INDEX. 


345 


Pleochroic  haloes,  31 

Pleochroism,  18 

Plutonic,  23 

Pneumatolytic  (Bunsen),  26 

Poecilitic  (G.  H.  Williams),  90 

Polarization-tints,  15  ;   table,  17 

Porcellanite,  276 

Porphyrite  (Neumann),  118,  126 

Porphyritic  structure,  27,  102,  151 

Porphyroide,  326 

Porphyry,  118,  121 

Predazzite,  308 

Proterobase  (v.  Giimbel),  131 

Pseudoporphyritic  (Harker),  90 

Pseudospherulite  (Kosenbusch),  109 

Pteropods,  254 

Pulaskite  (J.  F.  Williams),  52 

Pumice,  156 

Pyroclastic,  271 

Pyrome'ride  (Monteiro),  161 

Pyroxene-gneiss,  333 

Pyroxene-granulite,  333,  334 

Pyroxene-rock,  71,  85 

Pyroxenite,  50,  71,  85. 

Quartz-basalt,  196 
Quartz-ceratophyre,  104,  115 
Quartz  de  corrosion,  333 
Quartz-diorite,  57 
Quartz-felsite,  104 
Quartzite,  223,  230,  236,  294 
Quartz -porphyry,  104 
Quartz  sillimanitise',  292 
Quartz-syenite,  45,  49 

Kadiolaria,  285 
Reaction-rims,  77,  321 
Bed  clay,  242 

Refractive  index,  4;  table,  6 
Rhabdolith  (Huxley),  260 
Rhomb-porphyry  (v.  Buch),  124 
Rhyolite  (v.  Richthofen),  154 
Riebeckite,  105 
Rockallite  (Judd),  117 
Rock-flour,  240 
Rock-salt,  282 
Rutile-needles,  239 

Sagenite,  245 
Sand-grains,  224 


Sandstone,  223 
Saussuritization,  320 
Saxonite  (Wadsworth),  86 
Scapolitization,  320 
Schiller-structure,  317 
Schistosity,  237,  318 
Schorl-rock,  41 
Scopulite  (Kutley),  158 
Scy elite  (Judd),  94 
Sedimentary  rocks,  222 
Sequence  of  crystallization,  24 
Sericitization,  320 
Serpentine-rock,  97 
Serpentinization,  74,  98 
Shale,  237,  244 

Shimmer-aggregate  (Barrow),  292 
Shonkinite  (Weed  and  Pirsson),  50 
Shoshonite  (Iddings),  209 
Siderite,  262 
Sideromelane   (v.  Waltershausen), 

274 
Silicification  of  rhyolites,  159 ;  of 

limestones,  264 
Sillimanite,  291 
Sinter,  285 
Slate,  237,  244 
Slaty  cleavage,  240 
Soda-rhyolite,  168 
Sodalite-syenite,  56 
Solvsbergite  (Brogger),  122 
Solution-planes,  317 
Spectral  polarization  (Blake),  315 
Spheroidal  structure,  34 
Spherulite,  109,  159 
Spilosite  (Zincken),  300 
Sponge-spicules,  270 
Spotted  slate,  298,  300 
Sprudelstein,  256 
Staurolite,  292 
Straight  extinction,  8 
Strain-shadows,  315 
Strain-slip-cleavage  (Bonney),  241 
Sussexite  (Brogger),  126 
Syenite  (Werner),  44,  47 
Syenite-porphyry,  118,  121 

Tachylyte  (Breithaupt),  197,  206 
Tephrite  (Cordier),  210 
Teschenite  (Hohenegger),  139 
Theralite  (Rosenbusch),  83 


346 


INDEX. 


Thermal  metamorphism,  287,  290 
Tholeiite  (Steiniuger),  192 
Tiefengesteine  (Rosenbusch),  102 
Till,  240 

Tinguaite  (Rosenbusch),  125 
Tonalite  (v.  Rath),  62 
Tourmaline-granite,  41 
Trachydolerite  (Abich),  181 
Trachyte  (Haiiy),  170,  174 
Trachytic  structure,  47,  174 
Trappgranulit,  334 
Trichite  (Zirkel),  158 
Tripolite,  285 
Troctolite  (v.  Lasaulx),  85 
Trowlesworthite  (Worth),  41 
Tuff,  272 
Twinning,  10 

Umptekite  (Ramsay),  53 
Undulose  extinction,  315 
Uralitization,  60,  79,  132,  321 


Variolite,  197,  207 
Vesicular,  156 
Vitrophyric,  106,  152,  197 
Vogesite  (Rosenbusch),  141,  147 
Volcanic  dust,  271,  275 
Volcanic  rocks,  151 
Vulsinite  (Washington),  176 

Websterite  (G-.  H.  Williams),  85 
Wollastonite,  292 
Wyomingite  (Cross),  217 

Xenomorphic,  23 

Yogoite    (Weed    and    Pirsson)  = 
monzonite,  50 

Zircon-syenite  (Hausmann),  51 

Zoisite,  293,  320 

Zonary  banding  in  felspars,  15 


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fills  a  serious  gap  in  the  botanical  literature  of  this  country. 

Outlines    of   Vertebrate    Palaeontology  for  the  use  of 

Students    of    Zoology.     By   A.    S.    WOODWARD,    Assistant 

Keeper   in    the    Department    of    Geology   in    the    British 

Museum.     Demy  8vo.      1 4s. 

Athenaum.     The  author  is  to  be  congratulated   on  having  produced  a 

work  of  exceptional  value,  dealing  with  a  difficult  subject  in  a  thoroughly 

sound  manner. 


Press  Opinions. 

BIOLOGICAL  SERIES. 

Lectures   on    the    History    of   Physiology   during    the 

sixteenth,  seventeenth  and  eighteenth  centuries.     By  Sir  M. 

FOSTER,    K.C.B.,    M.P.,    M.D.,    Sec.    R.S.,    Professor    of 

Physiology  in  the  University  of  Cambridge.       Demy  8vo. 

With  Frontispiece.     Price  9s. 

The  author  has  chosen  for  treatment  and  developed  certain  themes 
connected  with  the  history  of  physiology,  and  has  woven  into  the  story 
of  ideas  the  stories  of  the  personal  lives  of  the  men  who  gave  birth  to  those 
ideas. 

Nature.  There  is  no  more  fascinating  chapter  in  the  history  of  science 
than  that  which  deals  with  physiology,  but  a  concise  and  at  the  same 
time  compendious  account  of  the  early  history  of  the  subject  has  never 
before  been  presented  to  the  English  reader.  Physiologists  therefore  owe  a 
debt  of  gratitude  to  Sir  Michael  Foster  for  supplying  a  want  which  was 

widely  felt no  higher  praise  can  be  given  to  the  book  than  to  say  that  it 

is  worthy  of  the  reputation  of  its  author. 

The    Soluble    Ferments    and     Fermentation.      By    J. 

REYNOLDS  GREEN,  Sc.D.,  F.R.S.,  Professor  of  Botany  to 
the  Pharmaceutical  Society  of  Great  Britain.  Demy  8vo. 
Second  Edition.  12s. 

Nature.  It  is  not  necessary  to  recommend  the  perusal  of  the  book  to  all 
interested  in  the  subject  since  it  is  indispensable  to  them,  and  we  will  merely 
conclude  by  congratulating  the  Cambridge  University  Press  on  having  added 
to  their  admirable  series  of  Natural  Science  Manuals  an  eminently  successful 
work  on  so  important  and  difficult  a  theme,  and  the  author  on  having  written 
a  treatise  cleverly  conceived,  industriously  and  ably  worked  out,  and  on  the 
whole,  well  written. 

PHYSICAL  SERIES. 

Mechanics  and  Hydrostatics.  An  Elementary  Text-book, 
Theoretical  and  Practical,  for  Colleges  and  Schools.  By 
R.  T.  GLAZEBROOK,  M.A.,  F.R.S.,  Fellow  of  Trinity  College, 
Cambridge,  Director  of  the  National  Physical  Laboratory. 
With  Illustrations.  Crown  8vo.  8s.  Qd. 

Also  in  separate  parts.  Part  I.  Dynamics.  4s.  Part  II. 
Statics.  3s.  Part.  III.  Hydrostatics.  3s. 

Knowledge.  We  cordially  recommend  Mr  Grlazebrook's  volumes  to  the 
notice  of  teachers. 

Practical  Teacher.  We  heartily  recommend  these  books  to  the  notice  of 
all  science  teachers,  and  especially  to  the  masters  of  Organised  Science 
Schools,  which  will  soon  have  to  face  the  question  of  simple  practical  work 
in  physics,  for  which  these  books  will  constitute  an  admirable  introduction 
if  not  a  complete  vade  mecum. 

Heat   and   Light.     An  Elementary  Text-book,  Theoretical  and 
Practical,  for  Colleges  and  Schools.     By  R.  T.  GLAZEBROOK, 
M.A.,  F.R.S.      Crown    8vo.    5s.     The  two  parts  are  also 
published  separately.          Heat.     3s.          Light.     3s. 
Journal  of  Education.     We  have  no  hesitation  in  recommending  this 
book  to  the  notice  of  teachers. 

Practical  Photographer.  Mr  Glazebrook's  text-book  on  "Light "  cannot 
be  too  highly  recommended. 


Press  Opinions. 

GEOLOGICAL    SERIES. 

Handbook  to  the  Geology  of  Cambridgeshire.  For  the 
use  of  Students.  By  F.  R.  COWPER  REED,  M.A.,  F.G.S., 
Assistant  to  the  Woodwardian  Professor  of  Geology.  With 
Illustrations.  Crown  8vo.  7s,  Qd. 

Nature.  The  geology  of  Cambridgeshire  possesses  a  special  interest  for 
many  students.  ...There  is  much  in  Cambridgeshire  geology  to  arouse  interest 
when  once  an  enthusiasm  for  the  science  has  been  kindled,  and  there  was 
need  of  a  concise  hand-book  which  should  clearly  describe  and  explain  the 
leading  facts  that  have  been  made  known.... The  present  work  is  a  model  ot 
what  a  county  geology  should  be. 

The  Principles  of  Stratigraphical  Geology.  By  J.  E. 
MARK,  M.A.,  Fellow  of  St  John's  College,  Cambridge. 
Crown  8vo.  6s. 

Nature.  The  work  will  prove  exceedingly  useful  to  the  advanced  student: 
it  is  full  of  hints  and  references,  gathered  during  the  author's  long  experience 
as  a  teacher  and  observer,  and  which  will  be  valuable  to  all  who  seek  to 
interpret  the  history  of  our  stratified  formations. 

University  Extension  Journal.  Mr  Marr  is  an  old  University  Extension 
lecturer,  and  his  book,  which  is  distinguished  by  the  lucidity  and  thorough- 
ness which  characterise  all  his  work,  cannot  fail  to  be  of  service  to  University 
Extension  students  who  are  making  a  serious  study  of  Geology. 

Crystallography.  By  W.  J.  LEWIS,  M.A.,  Professor  of 
Mineralogy  in  the  University  of  Cambridge.  •  Demy  8vo. 
14s.  net. 

Atlienaum.  Prof.  Lewis  has  written  a  valuable  work. ...The  present  work 
deserves  to  be  welcomed  not  only  as  a  greatly  needed  help  to  advanced 
students  of  mineralogy,  but  as  a  sign  that  the  study  itself  maintains  an 
honoured  place  in  the  University  Science  Course. 

Nature.  The  author  and  the  University  Press  may  be  congratulated  or 
the  completion  of  a  treatise  worthy  of  the  subject  and  of  the  University. 

Petrology    for   Students.      An    Introduction   to    the   Stud 
of  Rocks  under  the  Microscope.      By  A.  HARKER,    M.A.5 
F.G.S.,   Fellow  of  St  John's  College,  and  Demonstrator  in 
Geology     (Petrology)    in    the     University    of     Cambridge. 
Crown    8vo.     Third  Edition,  Revised.     Is.  Qd. 
Nature.    No   better  introduction  to  the   study  of  petrology   could   be 

desired  than  is  afforded  by  Mr  Barker's  volume. 


Sontion:    C.    J.    CLAY   AND   SONS, 

CAMBRIDGE   UNIVERSITY   PRESS  WAREHOUSE, 

AVE   MAEIA  LANE. 

AND 

H.   K.    LEWIS,   136,   GOWER  STREET,   W.C. 
Medical  Publisher  and  Bookseller. 


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